Olly:
You really don't know what Google is? (sorry if you do but were too lazy to use it)
Anyway, google it. You will find:
0. Go here:
Global Entry
1. You have to create a login/password (duh)
2. You have to fill out a fairly LONG online app. Block 30 minutes to complete and have ready your passport, drivers license, birth certificate, etc.... Be ready to fill out a 5 year address history (reminds me of filling out airline apps a long time ago)
3. Here is where i go fuzzy, because i quit before this part.... An in person visit at one of their offices wherein you bring some of the required documents that you used in step 2. The reason I quit the app before arriving at this point is I do not current live in the USA, and was unsure if they had offices in the country I live in. They might, but I didn't think it was worth the effort for the few times I come back to the US.
4. Pay the 100 dollar fee with credit card.
If I still lived in the States, I would go ahead and complete the app.