How do foreign militaries get slots at the US Army Ranger School?

Allied Nation Students come from bilateral security agreements and theater security cooperation agreements between the US and that nation. The “slots” are generated at the strategic level. At some point the G3-5-7 at DA allocates them to the allied nation. MIL to MIL diplomacy is one of the highest payoff lowest cost forms of diplomacy.

I assume it’s a similair process to when we send service members to similair courses on other countries.

When US servicemembers attend courses outside of the US they are assisted through the local Embassy/Consulate. There is a Military Group advisory office at Embassies that assists with this type of thing.