Updated: Dec 2, 2019
A plain weave is the process of pulling the weft thread (horizontal thread) over the first warp thread (vertical thread), then under the second, over the third, and so on until you get to the end of the warp threads. I always start left to right and start my weave going over the first warp thread. This is because I can later weave in my loose end of thread easily. Starting by going under the first warp thread would cause the weave to look not as seamless.
On the second pass back, you are now starting opposite of where you ended. So if my weft thread ended over the warp thread, my next pass back would be going right to left and passing under the warp thread, then going over, then under, and so on until the first warp thread is met again. The basic weave continues on in this way over as many warp threads as you wish.
A trick to making this process quicker is to utilize your Shed Stick
Now you can quickly pass your weft thread, on a Stick Shuttle