If you mean attaching a saddle stype mount to an1100 or 11-87, then you merely push out the two trigger assembly pins that are directly above the triger and replace with the bolts provided.
These cross-pins can be pushed oout with any ssoft metal object, a brass munch is great, but a common nial will work fine. Eash pin has a smll groove around it and a spring secures the pin by riding in that groove. So as you push or tap the pin out it will release and move slightly, then sometime hold up in the other spring. You can tap them out from either side of the receiver. I often use the bolts provide to tap out the factory pins, this keeps everything in alignment.
I would guess they just push out ,they do a 1187 ..one suggestion i can make on the New mount ,,do not tighten the bolts up too much they will tend to squeeze the action in and could interupt the function of the action
They just push out.
It should not take much force, one of the pins should have a shallow dimple on one side. Thats to help keep your tool from slipping off and scraping the receiver and so that you push it out in the right direction, from left to right I think, so that the pin will exit on the side of the receiver that ejects the shell. I use a dull pencil, or sharpened dowel.
I agree with Dave, do not over tighten the bolts, use blue locktite and just snug them a little at a time, while working the action, eventually the action wont close properly, then back off a little untill it does and your set.