I just converted a World Sport to single speed.

I removed the derailleurs first. Next I removed the chain and cleaned it with solvent and dishwashing soap. I measured how long the chain needs to be to run the smaller chain ring and the 15, 16, or 17 teeth rear cog, allowing a little space to tighten the wheel axle in the dropouts. I shortened the chain (careful that I would be able to refasten with the chain pin on the right link.) Then I put the chain on the bike, installed the rear wheel, and I had my single speed. All in less than an hour.

Next, maybe I'll buy a new wheel and single speed cogs, or maybe I'll redish the wheel and replace the freewheel with a BKMX freewheel.

For now, I'm happy and maybe more than half cool.

However, the right rear dropout had to be filed out to allow horizontal adjustment of the axle to tighten the chain. That did honestly add a little more time.