Being a longarm quilter, the most important things are: teach them to put borders on correctly, easing them in, not just cutting a strip and sewing to the sides at lightening speed. This causes wavy, wonky borders and a wavy, wonky quilt, and a longarmer's nightmare. Also, tear off those selvages... this is what comes to my mind foremost. We have a beginning quilting class here in town and I get quilts from many new students, and they never explain to them about borders. I have a regular sample and hand out ready, and after the first wonky quilt, I teach them myself how to avoid this. I have had to take tucks of as much as 2 inches on each side!!! Horrors!