One way to make sure it is wide enough is to lie under the fabric on your side with one shoulder on the ground, the opposite up. drape the quilt over you and measure. If the quilt is not wide enough the edges will rise off the ground when you roll over or sleep on your side creating drafts. Adding tapered "flaps" of single layer of nylon along the edges works great to help keep the quilt "tucked in".
To stuff in the down, I used a small bathroom, hung the baffles vertically on the towel rack, used small binder clips to close them when full. I just used my hands with the bag of down on a scale so I could see how much weight I was using. If your hands are sweaty the down will stick to them. Use climbers chalk to cover your hands, so the down will not stick to them. Move slowly. Sure down will float away, but it is easy enough to pick up off the floor later. Here are some pics. The black quilt was a Neatoman design with a foot box added for warmth.
If you use masking tape along the baffle sew lines, try to have the tape on the edge that will end up outside the "tunnel of the baffle, so that the tape is easier to remove. Just keep an eye out for that as you sew.
Lay the outline of your quilt on a floor with masking tape, or on a table, like a ping pong table. That will give you a guide for your cuts. You can do it for the baffles, too, ie. you will have tape going across the pattern along the lines where you will sew on the baffles.
Remember, when you read the directions all at once, it can be confusing and discouraging. But, when you go step-by-step as you sew, the directions become much more clear and understandable.
Go for it. Down packs down much smaller than synthetic, taking up less room in your pack.
Sleeping bag, next?