You're settled in the UK. That means you qualify to file directly at the US Consulate in London. This saves you time.
You don't apply for a green card directly. What you apply for is either a CR-1 (married less than 2 years) or an IR-1 (married more than 2 years) visa (same application, same process, you don't need to worry about which). Upon activation of the visa, a green card is issued, which supercedes the visa.
You'd fill out and file the form I-130 (along with a G-325A biographic data form) at the consulate. That's the petition to apply. Once your petition to apply is approved (about 10 weeks, give or take, and much faster than Stateside processing), you receive a paper called the NOA-2 (Notice of action) and the papers are sent to the immigrant visa processing unit. Your husband will then receive his invitation to apply, along with the actual application form (DS-230), IV-15 checklist telling him what documents to collect, an I-864 for you (financial support form) and information on how to schedule the medical exam.
Once the documents are gathered, the medical scheduled and you're ready, you send back the DS-230 and IV-15 checklist. Your interview is scheduled.
On the day, you bring the documents to the Embassy. They give you a number. You're called up to hand in the papers. You sit back down again and wait, wait, wait. You get called again, your husband answers questions on the paperwork and you're given the decision. If the decision is good, you go over to the payment desk and pay for delivery. If the decision isn't resulting in a visa, they'll tell you the next steps.
If you do receive the visa, then when he enters the US a green card will be produced for him. If you enter on a CR-1, you get a 2 year green card that needs renewal. If you enter on an IR-1, you get the 10-year green card.
That's it in a nutshell.