Always wear gloves and safety glasses when working with PVC glue / cement. If you happen to get some of the glue on your skin, lotion works well for removing it. It will take some time to remove the glue. Simply apply the lotion to your skin and rub it into the affected area.
Primer needs to be applied to all fittings that are pressurized. Primer may not be necessary for lateral lines (lines that are not under pressure, which are installed after the valve). If you wish to skip the primer application, make sure the glue you are using is rated to be used without primer.
I always apply the primer to the pipe first. Continue applying primer until the pipe is clean and softened (if you can still see the printed label on the pipe, you haven't applied enough primer).
Do not apply primer past what the fitting covers. Primer is intended to clean and soften the pipe, making a stronger bond between the pipe and the fitting.
Apply primer to the inside of the fitting socket. The fitting needs to be assembled while the primer is still wet, if it has dried completely, the joint won't be as strong.
Apply the glue evenly to the pipe. Do not apply the glue past the point that the fitting will cover.
Apply the glue to the inside of the fitting.
Press the fitting and pipe together, twist up to a quarter turn, if you are needing to align your pipe. If you are able to align everything without twisting, the fitting will be stronger. Hold the fitting together until the glue has set up enough to keep it from pulling apart. This may take only a few seconds in hot weather, but could take several minutes in cold weather.
Wipe off the residual glue. Congratulations, you have just welded your first PVC fitting. Yes, the proper term is PVC welding, when done correctly, the pipe and fitting are fused together, not just glued.