I started mines just before christmas when the 1.6 went tits up.
I've been doing it myself, after work, on the drive, in the snow and rain... not ideal, but m'eh!!
Best advice I can give is as follows:
1) Get a haynes manual, read it, but by all means don't worry if you can't follow it to the letter

2) Definitely get some mates round, doing it by yourself is a right royal pain in the arse!
3) If you have room to do it inside, do it.
4) Before you start and take the car off the road, take it to someone with a steam cleaner and get them to give the engine bay a proper good clean, as removing crappy oilly crap is not nice, and the cleaner it is the less soul destroying it is

5) Print out the various Si to 2.0 ltr, XR2i to 2.0ltr etc guides so you have them in the garage with you.
6) Take you time, enjoy the process, and if you get stuck, take a step back read the bit of the guides and see if you can think through it. If not, just come on and ask, someone will have done the exact same thing as you before

7) Plan to have the car off the road for a couple of days longer than you think it'll take, just in case.
8) Make sure you have a decent tool kit. I bought the Halfords Advanced 152(i think) one which is half price at £100 at the mo, and the only extra tools I've needed is a torque wrench, clutch alignment tool, cam timing kit and a smaller Inverse Torx bit for the exhaust manifold studs.
The whole process is a simple nuts and bolts job as Paul implied, and if you can hold a spanner and think about how an engine works, you'll be laughing

That said, I haven't tried turning mines over yet so I reserve the right to come back and change the above as it may be utter bollox

Paul