Watson

What's the worst that can happen? It's just fabric and it's likely a good idea to use cheaper fabric to start out with.
I try to make my quilt practice sandwiches (I usually use about a fat quarter ), ideally out of one side light and one side dark and use a light thread. That way you can see on the dark side how your stitches look and on the light side how it would look with matching thread on an actual quilt.
I always suggest you begin with a box, like you have and start with one motif and really nail it. Mark your sandwich into 4 boxes. It might be meandering or my favourite, paisleys, or loops like making a big lower case e and then a small lower case e, or anything you think feels natural to you. Some people draw it out, I can't draw worth spit, so that doesn't help me, but it might you.
If your machine has a speed control, find out what is a good speed that your hands can move comfortably with and then set the speed control to that. Then, you can push the pedal all the way down while you FMQ and only concentrate on your hand movement and the machine will keep the same speed for you.
While I was learning to meander the best advice I got was that if I got stuck about where to go next, I should stitch balloon capital letters like T and H until I got going on my own again.
Above all, don't be afraid to stop and think about where you are going next if you need to. Those people you see in the videos have been doing this a long time so they have a lot of muscle memory and experience....it will take some time to figure out how to get into and out of corners etc.
Also, once one sandwich is done, you can just baste another piece of fabric on top and use it again one more time, although you won't be able to see your stitches on the back, but you save a bit of batting and backing.
Leah Day is great for watching her do the same pattern over and over. Check out her Paisley pattern. You might like it and it teaches you a lot of techniques in one go and is a great filler.

Watson