1 - Be sure to measure first, then cut your borders to measure before sewing on.
2 - Sandwiching can be done in many different ways. I recommend heavily starching the backing before layering, and also spray starching the top before layering. Starching stabilizes the fabric so you have less chance of puckers when machine quilting. I also recommend spray basting because it holds the layers together better than other methods (and is faster). See old threads about how to spray baste.
3 - An easy way to quilt your first quilt would be to use the walking foot but *not* do stitch-in-the-ditch. Just make softly waving lines from one side of the quilt to the other. (SITD can be nerve-wracking and a perfectionist like me never does it well enough. Softly waving lines mean you don't have to be precise and gives a soft look to the quilting.)
4 - Attach binding after quilting.