Quantity   Component name
1 × Arduino Pro-Micro 5v 16MHz Clones work fine in my experience. The important thing is to have the ATmega32U4 chip as it is USB MIDI capable. Also ensure you have the 16MHz version as this is required by some of the libraries.
1 × Extension Cable for your Controller This is where the builds will differ, but just grab a cheap extension cable for whichever system you are building.
1 × Logic Level Converter The one in my diagram is a common bi-directional converter, but you can adjust to use a uni-directional one if you want.
1 × 5v to 3.3v Voltage Regulator Module Common 3 pin regulator module. You could always build your own regulator circuit, but these things are so cheap I'd say just grab one.
3 × 220 Ohm Resistor Resistors for MIDI and the Indicator LEDs. You can use any value up to 1-2k for the LEDs and be just fine, I just find it easier to use one value for all the resistors.
2 × LEDs (Different Colors) I use Green for Drum Mode and Blue for Synth Mode, but use whatever colors you'd like.
1 × Momentary Pushbutton (Normally Open) Use any size/flavor you want.