heh, if you have to ask how hard it is, it's probably too hard for you

The following is a list of things you have to, or is highly recommended that you do:

Change out the stock fuel pickup with an Si Unit, put in an in-tank fuel pump [or save the hassle and swap over an entire Si tank.

Change out the fuel hardlines that run under the chassis, the stock lines aren't rated for the pressure, and I believe are different sizes.

Swap out chassis electrical harnesses, or be prepared to read a lot of wiring diagrams and wire it yourself. Same goes for the engine bay harness.

If you're interested in having the PGM-FI status light in the dash, the easiest way is to swap dashes.

I may be missing some things, but that's a pretty good idea of what you're up against. Basically, you're best off starting with an Si, or getting a mostly complete Si for parts, have quite a bit of time, and even more patience.