Which way do put a crank on a bmx?

Answer 1

This question is a bit unclear, it doesn't tell if you're:

A) just talking about the crank arm,

or:

B) if you want the chain to go on the left side

If it's A, then it depends on which type of bottom bracket you've got, it's different if it's cottered, square taper, splined, or BMX style. Cottered cranks use wedges to hold them on, square taper use bolts that go into the BB axle. Splined can use either a square taper style axial screw or a pinch bolt. BMX tends to use pinch bolts. Then there are cranks where the left crank arm is permanently bonded to the bb axle

if it's B, it's not really advisable to use a right sided crank on the left side of the bike. There's something called precession, which tends to cause the pedal to come off.

Left side cranks use a counter-clockwise pedal thread to avoid the problem.