If the trigger and disc are in, stick the hammer pin just started in the hole on either side.
Put the spring on the hammer, with the legs sticking down.
The legs are supposed to rest on top of the trigger pins, and one leg will catch in the groove of the trigger pin, retaining it.
So, with the hammer held straight up, hook the legs behind the trigger pin, and push it up toward the front and down, so that the legs end up on top of the trigger pin about 90*. It will want to push back up and to the rear. But when you get the hole aligned, just push in the hammer pin.
As the pin goes through, you will have to tap it past the J-spring on the hammer by rapping the pin with a screwdriver plastic handle. Just a tap. Continue to push it on through, and you will have to wiggle the hammer around to get the pin lined up with the other hole. Push it on in.
All there is to it.