First step is to cut out all your pieces. I like to put my inner pieces grouped together and my outer pieces grouped together as each set needs something done to them before sewing the inner and outer layers together.
Now this part you have some wiggle room with placement. You can have your opening a bit lower than mine but I like to have the top edge of my welt pocket piece 3/4 of an inch below the top edge of the leak guard.
Fold the layer in half and snip in the center of the rectangle all the way to each edge. Take care not to snip the threads on the short ends.