1. Normally I dig out at least four feet of pipe to insert a fitting. The longer the better. I dig out 1 foot on one side of the fitting, and at least four feet on the other side of the fitting for the bending part.
2. Glue the fitting on one end first.
Are you inserting a Tee fitting for a swing joint, or a branch line? If you are branching the line, glue the Tee to the branch PVC first. That way the Tee fitting will be at the correct angle when you attach it to the first side of the existing PVC. You don't want the Tee fitting too high or too low (facing down). Then glue the Tee fitting and the branch line to one side of the existing line. Then cut the existing (last piece) pipe to the correct length. Lift the pipe up, hold it in the middle of the four foot section, and push the cut end down into the fitting.
Don't forget to bevel all cut sections of pipe before gluing.
3. I would tip the pipe ends into the fitting because I don't have a clue as to what you mean by bending a section of 1/2" PVC into an S shape. I think that would be pretty cool. Get pictures if you bend it into as S shape.
It's easier if you have help, if you don't you will probably want to lay on the ground to get close enough to use both hands for the bend. I normally lay a piece of cloth under the pipe so that soil doesn't collect of the glued fitting or pipe if I accidently touch the walls of the trench while bending.