For the water pump, there should be 4 brass screws to get the cover plate off. A white gasket between the cover plate and the pump housing - usually stuck to the cover plate when you take it off. Unless it rips, that white gasket is reusable if you did not get a new one in the kit. Twist and slide the new impeller. Once you line up the shaft to the impeller, it should slide in a with a little help. I also use a scotch-brite pad to smooth down in the inside of the cover plate - make sure you do not have grooves. Do not overtighten the brass screws when putting back in. Be careful not to drop anything under the motor b/c t's a PITA. Use a towel or box to catch anything. Dawn soap if your kit did not have some lube also helps.
Depending on your brand, there may be additional parts not used in your kit. I know Wakemakers sells the Johnson kit that come with two black gaskets not used in our application.
https://www.wakemakers.com/products/...water-impeller
For the ballast pumps, like a jabsco, there are 3 brass screws to get the cover plate off. A black o-ring that sits inside of the pump housing - sits in a groove. Try not to loose that, it can also be reused, but some kits come with a new one. The shaft has a slot to line-up and just slide it back in. Again, do not overtighten the brass screws.
https://www.wakemakers.com/collectio...mp-service-kit
I'm sure there are some YouTube videos floating around out there.