You need to modify the piece before you slipcover. Things like undersized arms and loose back cushions can make your slipcover look messy. You can build up undersized arms with upholstery foam or batting.
To keep loose back cushions in place, slip a long dowel or a broomstick through the top pocket of an old bed sheet. Lay the sheet flat over the cushions and tuck the broomstick down into the crevice of the framing. Then stretch the sheet back over the cushions and secure directly to the underside of the framing with a staple gun.
When you're finished with modifications, it's time to slipcover. Two tools you'll definitely need, some foam pool noodles and a wooden spoon.
Drape the slipcover over your furniture and start smoothing it out and tucking it into all the crevices, your wooden spoon will reach deep into those tight spots without jamming your fingers.
When you've got it all in place, squeeze those noodles down into the framing. This little trick will hold the slipcover in place so it won't pull out when you sit down.
Then just keep tucking and smoothing until you get it the way you want it! I'm Mrs. Fixit and it's just that simple!