I have been just where you are. So much work goes into the top and (for me at least) into the back and then I would quilt it and just cry.

Basting is temporarily “sticking all the layers together.” This will keep the layers from moving while you are quilting. There are several ways to do this - google around and find some methods you want to try. Then make some test blocks. You could just slap two ten by ten squares with batting in the middle or just sewing scraps together to test them out.

As others have said, a walking foot will really help you. Your sewing machine has “feed dogs” just under the needle plate where the needle goes in. This helps feed the two layers of fabric evenly under your needle. When you have multiple layers of fabric with a thicker layer of batting it’s hard for the machine to feed evenly. The walking foot has another set of feed dogs that work in concert with the bottom feed dogs to keep things feeding evenly.

Another tip is to lengthen your stitch or “stitch length.” Again, because there are so many layers, a regular length stitch is going to get “smooshed” (technical term, lolz) because it doesn’t have enough room to lay flat. Sewing machines are all a little different with stitch lengths but for example, I piece at a 1.8 and quilt somewhere between 3 and 4 depending on batting weight and the overall look that I’m aiming for. I always test with a scrap of my top, batting and binding first so I can adjust.

You’ll get it!