For about $30 you can buy a video from Al Knoch showing how seat covers are installed. If you use new foam, which I think you should, the new foam is actually a little over sized. In the video, you can see Al's guy use a bread knife to slightly trim the new foam bun down to size. In the video, you will also note that Al's guy has very large muscles between his elbow and hands. This is from many seat cover installs. It's a tough fit. If you use your old foam with the new covers, they very likely will be saggy. Mine were. I had to use new foam with new covers to get a good appearance.
mrvette's plastic bag to help get the covers on, is, I've read a good idea. Personally, I think I'd stay with hog rings instead of the zip ties. On a previous seat cover installation, 30+ years ago, I used nylon cord in a lot of places instead of hog rings/zip ties. Worked fine with never any problem. Currently, I 'm all hog ringed.