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📷. Weave the shed stick all the way across your warp threads. Now when you are weaving with the shed stick turn the stick so it is vertical and creates a gap between the lower and upper warp threads. This gap is also known as the shed, hence the shed stick name 📷
Now you can quickly pass your weft thread, on a Stick Shuttle📷, through the shed. Once you are finished with this pass, lay your shed stick flat or horizontal again and push it up away from your weft threads. Now take your stick shuttle and use the corner to pick up the warp threads in the opposite way from before, working your weft thread across your piece. For the next pass bring your shed stick down by your weft again and turn it vertical to continue your weave in the same shed stick, then weave back pattern. You get the drift.