Step 1: Materials
Empty coffee cup(s)
Enough coffee grounds to fill your cups
Oyster mushroom spawn
I'll assume you can manage to scare up some empty coffee cups. Ideally you want the little plastic lid part as well. It will make things simpler.
Coffee grounds are really easy to procure. You're gonna need a decent pile of grounds so the best way to get them is just to head to your local coffee shop with a 5 gallon bucket. Just ask them to throw their grounds into the bucket and tell them you'll pick it up in a day. Starbucks says on their website that they will always give you grounds if you ask. I've had great luck hitting the local indie shops. The people who work in those places tend to be cool with the project. You don't want to get grounds that have been sitting around because that will only increase the likelihood that mold will start to form.
The mushroom spawn is something that you will probably have to buy. I would recommend just buying a block of espresso oyster spawn from Fungi Perfecti here: