Skoot, they are a little tricky to work in and yes they might seem a little big. First off make sure they are clean and dry, slippery does not help at all when inserting these.
Start by getting one side of the oring into the groove, I will usually hold the cap with my left index finger blocking the hole out the back side (cap not on tank tube) and use both thumbs to kind of work back and forth until all but a tiny bit of the oring is in and the little that is left is kind of folded to a point sticking out the top. Then pop this last bit in, this is where holding a finger over the back side helps so it doesnt just pop the other direction.
Once the oring is in I will take something (I use the hex shaped handle of my oring pic) and rub around to get the oring seated well and even. Two things to watch for is you want to minimize twisting the oring or getting it bunched to one side or the other as either of these conditions will make a hard spot in the oring that will have more resistance than where the oring is relaxed. Not critical I am sure but little things I look for when I am installing them.
After doing a few thousand of these you will be able to do them in your sleep
