Prescouting in a nutshell:
1) Figure out where the teams attending your event(s) are competing prior to said event(s), if at all.
2) Find webcast and match schedule of said prior events. (Being there in person is better, but that often isn't going to work out.)
3) Sit down with your existing scouting system and hope the webcast(s) are decent. Plan to stay all day (if you don't have other, more important things--
like school). You're also testing your own scouting system here.
4) Wait a day or two and check Youtube and TBA for videos of matches you want a better view of (probably most of the ones with teams you're interested in).
5) Find reveal videos of anybody who didn't compete.
6) Pit scout all day Thursday....
7) ...and throw out all the prescouting for several teams on Friday morning because they completely changed their robot between the event you prescouted and the event you're playing against them in.

__________________
Past teams:
2003-2007: FRC0330 BeachBots
2008: FRC1135 Shmoebotics
2012: FRC4046 Schroedinger's Dragons
"Rockets are tricky..."--Elon Musk
