The harp is the size of the space froom the pillar to the needle. At least 7 inches works best, but some amazing quilts have been done on smaller machines. You need to drop or cover your feed dogs and purchase a darning foot if you don't have one. Make some sandwiches about 12" by 18" with muslin and batting. Load the machine with a contrasting thread and start practicing. Some people have to adjust the tension on their machine. I don't on my 1956 Singer 301.
For me, the hardest part was convincing my brain that's it's okay to sew backwards. Once I got past that, things improved considerably. Use the index finger of your writing hand and make swirly patterns on a slightly rough surface - even the muslin will do. I use inexpensive Fons and Porter gloves to quilt, but not for the brain imprinting.
I am still in the practice, practice, practice stage, but was pleased to do flowers on my last quilt. I've only done large lap size, no bed quilts.
Edit: FMQ is much easier than SITD. There's no room for erro in SITD. Free motion is much more forgiving. There is a SITD foot available for most machines. It made a huge difference. I have only been quilting since last Fall, so my advice may not be good, but at least it's fresh.