Idiot’s guide to fixed gear conversion please

I’ve got an old road bike. It has a square taper BB in it. The rear wheel has a screw-on (Falcon) freewheel.

I know I’ll need a special tool to get the freewheel off. WHat will I find when the freewheel’s off? Is the axle threaded? Do I just screw on a suitably sized fixed sprocket? Where will it sit in relation to chain line? I can measure the length of the BB axle but I’ll then need some cranks – how do I get the right cracks to line up with the rear sprocket?

It’ll need either track ends or forward facing dropouts…have a look herefor the rest