Make a list of the electrical loads you will have.
This will require you to do a layout of machines and equipment. Don't forget loads for any electrical heating or air conditioning, exterior power, and every possible present and future source of power usage.
You will need to do a lighting layout. Don't forget exterior lighting.
Then you need to determine the required circuits. Some things will have dedicated circuits and some circuits will be lightly loaded, especially if you want to switch lighting circuits so you can turn the lights on in banks.
From the list of circuits, you will know how to size the subpanel and the breaker in the main panel, that will serve the sub. Make sure you balance the loads in the subpanel and also locate the breaker in the main panel, that serves the subpanel, so the main panel is balanced as well.
Part of the design of the system will include planning the routing of circuits. You will need to determine the best location for the subpanel, based on the main panel location and where all the circuits run.
Then all you have to do is buy all the materials, including ground rods, and install it.
Simple, eh?
Oh, forgot to mention. Don't forget to pull permits and follow all the requirements of the latest NEC and any other rules your local jurisdiction has adopted. Have you ever read a code book? It's great fun! Buy a good supply of headache pills.
And use all the standard techniques that a good electrician will learn and use.
All that being said, it's not rocket science. But you will have to acquire all the needed knowledge and techniques. And not being an electrician, you won't know what you don't know. Your best bet is to read a lot of forums and have an electrician friend to ask questions of. Sometimes an electrician at a big box store can help.
You need to choose the right size and type of feeder wires and install them correctly. Locating and mounting the box properly. Running circuits and installing protective plates. Fire-stopping holes. Sizing circuits. Choosing outlet and switch quality level. Deciding on panel brand and model.
All things to know.
Youtube videos:
https://search.yahoo.com/yhs/search?p=installing+garage+subpanel&ei=UTF-8&hspart=mozilla&hsimp=yhs-001
Anyone else have things to add?