Congratulations on your PC420, I have one and I love it. I have FMQ'd a lot on it and really enjoyed it. I just upgraded to a used PQ1500s for the larger throat space so that has become my main FMQ machine, but I still use my PC420 everyday for the decorative stitches. On to your question:
I pin baste. If I'm only using one color thread in my quilt when I'm FMQing, then I FMQ stitch in the ditch just around the part I'm getting ready to FMQ, remove pins and FMQ--it goes really quickly this way and I don't have any problems with tucks, layers shifting or pins getting in the way of my FMQ.
If I'm planning on using lots of different colors of thread in my FMQ then I pin bast and using a thread that blends or invisible top thread I Stitch in the Ditch as much of the quilt as I can get through my machine using a regular open toe satin or decorative stitch foot with my feed dogs up. Remove quilt, remove all pins from areas that were STID'd and FMQ. This way takes a lot longer since I'm also breaking a lot of thread to accommodate color changes, but it allows me to FMQ some really large areas without running into pins.
For learning FMQ, try starting with two pieces of felt for your sandwiches, you can buy large packages of felt from craft stores and just grab a couple of pieces and go. Then move to quilt sandwiches that you might want to use for other things like place mats before moving to a full sized quilt.
Rob