You need to understand the hardware and software you are building. College or formal education helps most of us. Some people have 'the knack' and are able to self-teach. Myself, I needed college, it really speeds up how much I could learn.
Then you need to read through datasheets for microcontrollers and pick the one that you think is going to do the job and be able to run the software you want to write.
Then you gotta build the hardware, so you need to do PCB (printed circuit board) design using a CAD software (computer aided drafting) such as EagleCAD, Altium Designer, etc.
Then you send the PCB design to a PCB manufacturer (there are hundreds, google it), they send you a prototype, then you have to purchase all the parts to put on the PCB from places like mouser.com and digikey.com. Then you have to buy soldering equipment from a place like allspec.com. Then you have to solder all the parts onto your prototype.
Then you have to make sure your hardware works and try to connect it to your computer using the method suggested in the datasheet for that microcontroller from the company who makes the microcontroller.
Then you need to write some test software that you can load into your hardware and help you test out your PCB design and hardware and everything to make sure you didn't screw anything up during PCB design.
It's an iterative process that takes a lot of experience and time. You are much better off just purchasing a 'development kit' from
www.sparkfun.com if this is your first project. Basically the 'development kit' helps you skip the difficult part of building and testing your own hardware They send you hardware that already works, with some software examples, and they send it with cables or whatever you need to hook i tup to a windows computer.
As you can see from this short description, there is no real way to answer you rquestion. If you are asking how to make a mod chip, then you do not have the skills to make a mod chip. You can't be taught how to build one and there is no easy schematic. Think of what inventors do - hours in a workshop, in the dark, hours a day working on their shit - this is how you make a mod chip. Most people are either formally educated or genius self-taught. Once you have the basic skills google can help you some, but in my opinion there's no replacement for formal education. If you're serious about a building microcontroller projects as a life hobby, you'll have to spend some serious time self-teaching or it may be best to try to go to college for it.