How do you reattach a bicycle crank arm?

Depends on the design. Some use wedges called cotters to stay in place, others use pinch bolts to clamp around the axle, some use a crank bolt to pull themselves onto a wedge shaped axle.

You need to use a technique that's appropriate for your design.

If it's a square taper crank that's come off, be advised that they often get damaged by wobbling around, to the point where thay can't be made to stay in place securely afterwards. That case, you'd have to replace it.