Okay, you have 2 steps for finishing. Not hard at all! Take a deep breath and relax. Then fetch about a dozen bobby pins.
Do as your instructions say... fold the lower end, your cast-on edge, up to where you started the flap. Simple so far, right?
Now use your bobby pins to hold the two edges together along one side. Put them through both layers, about 1" from the edge.
Take a yarn needle. Thread about 24" of your yarn through it. At the corner where you doubled the piece back on itself, ie - the fold, pass the needle through both layers [from bottom to top]. Pull the yarn through until about 3" is left to the end of your needle yarn. Use this tip to tie a knot.
Bring the needle away from your work, down and again insert it through both layers, bottom to top, maybe 1/4" towards the open top and away from the fold. Pull the thread snug. Repeat this "whip stitch" until you reach the upper edge and run out of one of your layers - leaving the flap only.
Now take 3-4 stitches, bottom to top, in the same place. This anchors your sewing yarn and strengthens the place where the most stress will be. Then weave your yarn through the wrong-side of your work and cut it off. Oh, now you can take out those bobby pins. Ta-Daa! You just sewed one side!
Repeat for the other side.
There have been several posts on how to pick up stitches - do a search and see if those can help you in that regard.