Fitting piston rings by hand is the easiest way. Just gently hook one end
down onto the piston, and work your way around to get the complete ring onto
the top off the piston. Then gently work it down the piston. Obivously

I highly recommend a ring expander to install the rings on the piston
and a ring compressor to install the piston in the cylinder.

I tried to install rings on a small piston by hand and I promptly broke
the ring. That was on a model engine with CI rings. Obviouly steel rings
are tougher. On model engines I wrapped some thin brass around the
piston and used a cable tie to compress the rings enough to slide them
into the cylinder.