I am in the process of turning an old geared bike (actually 2) into a fixie. Its fairly easy. If you can use a spanner you can do 99% of it. I have had a few setbacks, either due to the difficulties of working with an old bike (siezed and rusted parts mostly) and of not having all the correct tools, but its been fun, rewarding, and not too hard in retrospect.

The biggest mistake I made was not planning, and penny pinching. Although, that was the plan. Origionally I turned the bike into a singlespeed, so I ended up getting a SS freewheel I won't need. I also broke 2 weeks (old ones) and replaced them with equally old ones I got for £10 second hand. In retrospect, it would have made more sense to buy new wheels. So, either plan the build meticulously from the start and save money, or, experiment with different setups and parts, and be aware it will cost a little more in the long run.

First thing to do is find a good frame with horizontal dropouts. I am not an expert, but I would suggest there is a reason that the cracked frame is in the trash?