Fill out form, form is in English (see FAQ and link to online form). You will need your last 3 passports (or whatever you have*).
Three days prior to appointment pay visa fee at Banco Popular.
Also buy 5-min pin to call to confirm appointment (not necessary, I just wanted to make sure I got it right).
Show up half an hour early. Proceed to the entrance closest to the corner (the rear one has a pretty long queue, but US citizens and visa renewals are at the front entrance, they have personnel there, you can ask).
Once inside follow instructions. I had to be fingerprinted (before 9/11 it wasn't necessary to set foot inside the consulate to renew visas and I hadn't been fingerprinted)
Pay Domex (courier that delivers the passport when ready), you will be instructed where to go within the consulate.
I had to bring my passport (at least 6 months left), the receipt from Banco Popular and the copy of the confirmation you get when you finish filling out the form. That's that. The only question I (and everybody within earshot) was asked was "what is the longest you have been in US territory?".
Make sure you do not bring any items in their "forbidden list" (cameras, celphones, guns, etc.)
From the time I arrived (45 minutes early, but I was told I could go ahead and go inside) to the time I exited it took less than 20 minutes. I am not sure if that is the norm. My passport arrived about two weeks later by courier (he came unannounced, so I was lucky I was at home!).
I don't know what the process or requisites for a first-time visa are, I was renewing my 10-year visa (the third time renewing). Things have changed considerably since my last renewal 10 years ago, I had to go personally to do it whereas before I had somebody drop off my passport and pick it up.
*I had a passport stolen a long time ago, the consulate was notified at the time. I only have two passports (an old one and one that will expire early next year), but they needed to see them to check my traveling pattern.
Something I forgot: the form is pretty straightforward (assuming you are fluent in English), but one question had me baffled. It asked my "passport book number" (in addition to the passport number). A bit of googling revealed that the passport book number is printed at the bottom of page no. 3. It is the same number that is punched out in all pages, but followed by a letter (a Z in mine). You need the numbers and the letter. The passport number is at the top right of the bio page.
As I said, I was a bit clueless, things have changed considerably since my last renewal.