Necessary Items to begin developing modules, protoyping and doing more advanced diy modular projects - and even just basic electronics in general:

This List is p much the bare minmum you need to get into module development assuming you have the sense to prototype before sending off hard earned cash for pcb's that turn out not to work due to lack of testing - breadboarding, prototyping nad learning the easier/chepaer way that a circuit has a [fatal?] flaw that needs rectifying before full production or even just pcb prototyping stages is essential to the module development process.

not all of this is essential for basic diy but much of it will turn out to be very commonly used or extrenmly useful in regular diy project building and even stuff like an oscilloscope can be invaluable toaid in troubleshooting failed builds and repairing damaged modules.

you'll often have some of these things already and the rest you'd likely end up buying at some point in 
even a diy career and definitely in a module development career so you might as well take advantage of any deals, price breaks and sales etc that pop up and buy these items whenever you see a good opportunity, early on if possible.

***HINT: Simulators do NOT tell the whole story, dont give the full picture, arent real life and cant be relied upon - always at least make  atest pcb run and solder up a prototype yourself for testing before goin to full production runs at least, if you refuse to breadboard - you'll find so many things dont work quite right, are probne to the troubles of tolerances, real life physics vs. ideal computer situations and the issues of stuff like there not quit being true standards in eurorack or other modular formats, power being highly variable in actual voltage and stability and other things besides!***

TOOLS
- Side Cutters*
- Mini 125mm Long Nose Pliers
- Automatic Wire Stripper

- Adjustable temperature Soldering Station
- Desolder Pump/Desoldering Braid [I prefer the pump]
- Solder
- Stainless Steel Tweezers
- "Helping Hands" type Assembly Aid, with magnifying glass
- [optional: Handheld magnifying glass if you can't see the heccin resistor colour codes, & diode and capacitor labelling]

- Digital Multimeter: measurement of Vdc, Vac, Iac, Idc, Resistance, Capacitance, audible continuity [diode tester, transistor tester - optional]
- [optional: oscilloscope - I use a rigol ds1052e, possibly the most ubiquitous scope in the industry, nay the world!]

- IC Extractor Tool

*(note - the ones hospitals use for removing staples are spectacular, surgical steel, i've had mine 10yrs continuous 
use, sharp as anything perfect for flush cutting still!)


COMPONENTS

[Its quite common for electronics stores that sell learning kits or hobby store to sell starter packs for beginner electronics experiimentalists that contain a set of maybe 10 each of the whole E90 range of resistors and similar sets of capacitors, diodes etc - one of these packs can set you with your breadboarding needs in one relatively cheap investment straight off sparing the need for expensiv shipping, waiting and collecting stuff piecemeal.
Some of these beginners/intro packs will even have common basic ICs, LEDS, some breadbord and jump wires, soid core wire for joining hardwrae components like pots and sockets and other stuff to boot - do some hunting you may benefit hugely!]

ICs
cd4017b decade counter used for sequencers & more
cd4066b quad switch
cd40106b hex inverter with Schmidt triggers

LM13700 transconductance amplifier - a good way to cv stuff

TL072 dual opamp
LM324  quad comparator or MC4558 dual comparator

555 Timer multipurpose tool
PT2399 delay [echo]

RESISTORS
carbon film resistors
10R, 100R, 1k, 10k, 100k, 1M 10x [1/4W 5%]
15R, 150R, 1k5, 15k, 150k 10x [1/4W 5%]
22R, 220R, 2k2, 22k, 220k 10x [1/4W 5%]
47R, 470R, 4k7, 47k, 470k 10x [1/4W 5%]
56R, 560R, 5k6, 56k, 560k 10x [1/4W 5%]
68R, 680R, 6k8, 68k, 680k 10x [1/4W 5%]
82R, 820R, 8k2, 82k, 820k 10x [1/4W 5%]
1M5, 2M2 2x [1/4W 5%]

CAPACITORS
Ceramic Disc Capacitors 20x [+/- 0.25pF]
1pF, 10pF, 33pF, 47pF, 100pF, 470pF, 1nF, 210nF, 100nF

Electrolytic Capacitors, Radial
1uF 50V 10x [85 deg]
2.2uF 63V 10x [85 deg]
4.7uF 40V 5x [85 deg]
10uF 25V 20x [85 deg]
22uF 25V 5x [85 deg]
47uF 25V 10x [85 deg]
100uF 25V 10x [85 deg]
220uF 16V 5x [85 deg]

DIODES
1N4148 Signal Diode 4x [100V 300mA]

3.3V BZX55 Zener Diode 1x [500mW]
3.6V BZX55 Zener Diode 1x [500mW]
5.1V BZX55 Zener Diode 1x [500mW]
5.6V BZX55 Zener Diode 1x [500mW]
6.2V BZX55 Zener Diode 1x [500mW]
6.8V BZX55 Zener Diode 1x [500mW]
9.1V BZX55 Zener Diode 1x [500mW]
12V BZX55 Zener Diode 1x [500mW]

Transistors
BC547B Transistor 1x [NPN TO92] or 2N3904 [NPN TO92]
BC557B Transistor 1x [PNP TO92] or 2N3906 [PNP TO92]
BS170 N-channel MOSFET 1x [60V 500mA TO92] or 2N7000 N-channel MOSFET
BF245 N-Channel J-FET 1x [30V 6-15mA TO92] or J201 N-channel J-FET

HARDWARE
3.5mm Mono Jack Socket PCB mount, Vertical or Horizontal 10x
[Switched contact optional but wise because if no needed can be ignored but i ued v v commonly in eurorack].

Potentiometers - 9mm with round 6.35mm metal shaft. For pcb and/or panel mounting.
10k Linear 2x
10k Log 1x
25k Linear 1x
50k Linear 2x
50k Log 2x
100k Linear 5x
100k Log 2x
500k Linear 1x
1M Linear 1x

[optional: Selection of various Horizontal Finger Adjust trim pots.
trimmer pot adjustment tool.]

Miniature Toggle Switch SPDT, 2x [5A/125Vac 2A/250Vac.]
choose type as seems most useful: e.g. ON-ON, ON-OFF-ON etc.)

[a random assortment of other switche, minipots, buttons and sliders may well prove useful to your individual needs - buy what is relevant to ryour particular planned projects]

OPTO-ELECTRONICS
an assortment of whatever colour and brightness LEDs grab you for a sensible price
[RYO use 3mm amber mostly but occasionally green and red, in quite low brightness ratings, but you may desire other options - they'll just require slightly different value resistors in circuits where LEds are involved.]

i'd recommend about a selection of 10-15 various colour 3mm LEDs, any luminous intensity [mcd]

[optional: NSL32 - Optocoupler with resistive output.2X [Observe polarity]

H11L1M Optocoupler, High speed optoisolator with Schmitt trigger output. 6-pin DIL.]

PROTOTYPING
2x Solderless Prototyping Breadboard 172mm x 63mm, 840 tie points. [Multiple boards can be locked together for larger prototyping areas.]

Approx 100 male/male jumpwires for use with breadboards, various lengths with solid tips.

...

FOR POWER
You'll with need a lab/bench-top type psu, or alternatively, you can get a small eurorack psu like the tiptop happy ending kit [hek].

- The lab psu's have pro's: stable, accurate power to be dialled in at whatver voltage and amperage you choose

- The hek has the pro's: provides actual real life eurorack module power to see the effects of actual setups.
^ Bonus pro's: holds maybe 7-10 eurorack modules in a little desktop skeleton rack, allowing a set of vco/vca/eg/lfo/filter/etc as test modules!
^ (See how your breadboard circuits behave with and respond to real life eurorack modules!)


FOR RYO PROTECTED BREADBOARD POWER [optional]:

Solderless Prototyping Breadboard 80mm x 60mm, 390 tie points: 29 columns in 2 banks of 5 plus 4 rows of 25

- [if using an hek or similar to power the breadboard: 8-Way single red-stripe ribbon cable approx 30cm length,
female 16-pin IDC connector, (optional, and highly convenient: idc crimping tool).]

- [if using a bench-top/lab psu to power the breadboard: lab/bench-top PSU, Approx 30cm of 4 x Dif colour solid core wire to connect psu to the breadboard.]

2 x 72V 0.25A PTC resettable Fuses: [10r Resistors, carbon film 2x [1/4W 5%] will do in a pinch**]
3x 100nF:  Ceramic Disc Caps 5x [+/- 0.25pF]
2x 10uF:   Electrolytic Caps, radial 2x [85 deg]
3x 100uF:  Electrolytic Cap, radial 1x [85 deg]
LM2391:   Voltage Regulator 1x [TO92 package]
2x 1N5818 Diodes: Schottky Rectifier Diode. 2x [30V 1A 0.5Vf]

[** you may find these serve you better when starting out - they cost 10ths if not 100ths of cents, and when you screw up - you will, the bang and smoke of these blowing is much more of a lesson leaned than waiting for a PTC fuse to cool - especially if you burn your fingers trying to replace the 10r that burned out before waiting for it to cool fully... :D ]

The +12V, -12V and GND on the right hand end are what is inserted into the breadboard rails that run continuously down the length of most slderless breadboards and are commonly marked as gnd/+ve/-ve

The header at this end is non-essential for testing and prototyping - this can be any connection to a bench PSU, modular ribbon cable power source or whatveer you're using

The current RYO power inlet circuit for eurorack modules [+5V not present] - as you can see there are PTC fuses for high current protection and schottky diodes for reverse current protection as well as a variety of decoupling caps and hte shrouded header that triple secures plugging a module in backwards doesnt occur without real idiocy.