If you haven't picked up fabric yet here are things to think about.
You want a fabric that has a simple pattern. You do *not* want to try to match plaid pieces.
You do not want a fabric that has "nap". With a fabric with nap you need to be sure all pieces are in the correct direction. The same can be said with a printed fabric. Some fabrics have a print that can be looked at from either direction.
A simple fabric will make your job much easier because all you need to worry about is making sure the pieces are on the straight. The straight of the grain runs the length of the salvage. In other words don't cut pieces sideways as they will stretch out funny when you sit on them.
I would go to the library and get out a slipcover or upholstery book to get you going in the correct direction.
Sorry, if this is too basic. I don't know your skill level. Just wanted to put some things out there for you to think about. I really hate having to match up patterns and nap.
If you need to cut your foam the best tool is an old electric knife that we use to use for turkey day.
Another thought. I am not fond of putting in zippers. It is much more professional, but recently I had to make slip covers for condo cushions and I just used velcro. Now we will have to see if they "rip" each time someone sits on them. In my Scamp
I have also just "whip stitched" the final seam on one of my cushions. Whip stitch is sewing the final seam by hand.
Hope this helps some. Wait for sale
day at the fabric store. Shop around.