Make no mistake: installing an infant car seat can be strenuous work. You need to achieve a fair amount of compression for the seat to stay put. This will probably require sitting or standing on the seat while pulling hard on the belt strap. If it seems easy, you’re probably not putting the seat in firmly enough.
Besides adjusting your expectation of work, here are a couple of tips I’ve discovered over the years:
If you’re struggling to grip the belt, try wrapping it around a strong dowel, or even a large screwdriver. This will provide you a handle.
Be efficient! If you are not pulling up directly in line with the natural path of the belt from the buckle, you’re going to have to pull a lot harder for the same results. What I do whenever possible is pull the seat fabric out of the way to provide a path up through a gap in the seat, instead of going around the outside. The difference is dramatic. If there was one tip I wish I’d been told years ago, it would be this.
The above applies to toddler seats too, but they’re much easier to get right in the first place.