Earlier, Tugadude cited the late, much-missed Sheldon Brown's very helpful, illustrated and handy synopsis. That would be <https://sheldonbrown.com/fixed-conversion.html>, which is probably the UR-site for fixed-conversions and could be considered ground-zero for the whole fixed-gear boom earlier this century. Sheldon's writings launched who knows how many fixed conversions - and it's still good and relevant information to this day. There is a passage on using conventional freewheel hubs by redishing the wheel and simply threading on a fixed cog that explains the process nicely. Good luck and Godspeed.

Though, it must be noted - the Gitane Track Standard you also have blows this out of the water. Were I in your shoes, I would convert this one as quickly and cheaply as possible, because the Gitane is soooo vastly superior.