If you need to change the washers on your P-trap, then the best option may be to remove the entire trap and start again with a new one. Replacing parts of a P-trap can be very difficult, and you need to know what you are doing in order to get the trap fitted correctly. Follow the steps below to effectively replace the washers in your P-trap.
Step 1 - Removing the Old Washers
Use your wrench to unscrew the P-trap from the fittings. Do this by removing the trap from the trap arm, and then take off the other side. Look at the washers around the connections on the P-trap. You may notice that one looks very different from the others, or you may decide to replace them all.
Step 2 - Fitting on the New Washers
Place a series of washers, including a metal one and a rubber one, at the end of the P-Trap. These washers should be a similar size to the one already in the trap, so make sure that you have checked the size before you come to replace them. Use a little caulk around the edges of the threads to ensure that the seal is tight, and then replace the trap onto the rest of the plumbing. Don't tighten too much, as this could break the washers.