Building a midi controller!!

Hi guys! I want to build a simple midi controller to better control fx in ableton (in combination with the apc mk2). I've read the two posts here on djtt and some others, but I still have one or two questions. First, which specific parameters does the HID need? There's a lot of them on the market, starting from the Arduino's, but I don't know which specifics i have to research in order to build a controller. Then, what do I have to do to make my computer recognize the controller I build as a midi device? Hope some of you have already done it. Thanks!

For ableton effects, it works best if it is seeing a USB midi device, and it is receiving either MIDI CC messages from knobs and encoders or NOTE ON and NOTE OFF messages for buttons and pressing a "clickable" encoder. You just enable midi mapper, click on the thing you want controlled and then turn that encoder or click that button to get Ableton to learn it. There is also a small drop down menu in the bottom left that appears which lets you select what kind of encoding standard Ableton should be using if it doesn't learn it correctly the first time and starts acting weird when you use the control.

Apparently you can get Ableton to send note and CC messages back to the controller for things like activity LEDs but i've not had this working as it needs you to write mapping scripts which I don't know how to do.