Replace the gasket between the cylinder and air compressor if it isn’t intact.

Rotate one piston to top dead center.

Align the upper piston with its corresponding cylinder.

While holding the cylinder closely aligned with the piston, use one hand to compress the piston rings.

With the piston rings compressed, begin to lower the cylinder onto the upper piston.

If you have access to a ring compressor, it can be used, but is not necessary.