There are lots of YouTube videos and some people practice on dry erase boards to imprint designs in their brain before trying it on their quilt. All are good, but when you start to practice on fabric, the most helpful quilt sandwich to start on was a large coloring book style kids panel. It helped me learn "control" with FMQing on my machine. Outline the lines like we used to do when coloring. When all the lines have been followed, start filling in the spaces with different textures. When you are done, bind and donate. I tried doing quilting on a muslin sandwich in which I had drawn lines to resemble squares, triangles and borders, but quickly got bored. Here is one of my practice panels:
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Just relax and have fun. Starting is the hardest part. I use Machingers gloves as my hands slip otherwise. I position my hands to the left and right of the needle, but sometimes, like Geta Grama on this board, will pinch a bit of the fabric from underneath with my right hand instead. There are no hard and fast rules, so do what is comfortable for you.

I was so afraid of ruining the top that took forever to finish, I traced the designs I wanted onto tracing paper and pinned them to the quilt and sewed through the paper!! Then when done with a section, rip the paper off and sometimes used a tweezer to get all the paper!! That's when I started using chalk pencils to mark the design, both have their drawbacks!
Just relax and enjoy the process!

https://leahday.com/pages/quilting-design-gallery This link will take you straight to her stitch gallery; click on a FMQ pattern you like, and you can see a how-to video on it. She has literally hundreds of these instructional videos. It's an amazing resource.