Congrats! Be proud of your work.....I still have and use my first...over20 yrs ago-with 1/2" seams!
As suggested do simple quilting, but don't allow the max emptiness suggested on batting info....that will over time cause the batting to get lumpy with washings...."old" rule of thumb was hand width size (across fingers) largest amount not secured with quilt stitch. And when using walking foot, lower your foot pressure to about 2-3 and stitch length to about 3... This will allow easier movement of sandwich. Since you've never done this before, I recommend you make a small sandwich about the size of an old fashioned cloth diaper, and get the feel of machine quilting...you can even get bold, lower your feed dogs, but on the correct foot and try some free motion - just for kicks and grins!