The Karsten is a simple fit. Drill two holes (if you go for that model) and job done. I've done this on a few rifles over the years but there is now a velcro model that simple sticks to your stock.

The three way butplate is a little harder. There is the issue of making sure the plate matches the shape of the stock, you have to drill into the stock in such a way that the butt pad works "straight". So takes more finesse than the cheek piece. Also with some composite stocks they may be hollow or the material is too hard/soft to properly fit the three way butt pate pictured above.

You might find alternatives in terms of spacers on the butt pad or using a more simplified version that is not adjustable in length, but only for camber and height. Like this: