I don’t suggest you start out machine quilting a large quilt. I think working on smaller projects will help you be more successful and an easier way to begin learning this skill.
This is a skill that everyone can master and the trick to mastering it is to practice. No matter how many books you purchase or classes you take you will never get very good at this if you don’t practice. I think you should make up some sample squares 15” AND keep them by your sewing machine and practice free motion quilting a half hour every time you set down to sew. You will eventually begin to understand the mechanics of holding the sandwich and moving the sandwich and moving your hands to match the speed of your foot.
When you are working on a DHSM (domestic home sewing machine) you want to hold the quilt with your two hands of if they were the sides of a hoop. You will be quilting for the most part away from yourself. You will be working from left to right and then down, and back again from right to left in a regular but free looking motion. You want to think about where you are going next with your quilt design and how are you going to get there. You will work in an area about the size of a dinner plate, and then readjust your hands and the quilt and work again the area about the size of a dinner plate.
When you work on a DHSM you need to match the speed of your hands to the speed of your foot to get even stitches and develop a rhythm. You need to think of the process as if you are moving the paper under a stationary pencil. You are creating the design by moving the paper under the pencil. On a long arm machine you move the pencil over the paper.
There are tons of books out there with lots of patterns. I have a self published book with great beginner patterns in it. Sit down and try each of the patterns until you become comfortable making them.
A great way to practice on a medium size project is to buy a cheater panel (you know a preprinted panel, like a kids scene) and layer it into a quilt sandwich and quilt this up and bind it and give it away to charity. This is a great way to practice on a medium size project.