For the snap traps I think the bait is key.
The perfect bait - irresistable to mice but not grab and run - is peanut butter coated in scratch grain. The scratch grain sticks to the peanut butter and WHAM!
The best location is close to, but not touching a wall. But with a big infestation as I had, you can get them in the center of the floor, up on the countertops and anywhere else you can imagine.
I don't really use my live traps any more and would be happy to send some to you - I was contemplating giving them to Habitat for Humanity but I suspect they won't be interested. I have a load of 'mice cubes' - very simple but very effective - and a multi mouse one that is also very effective.
https://smile.amazon.com/gp/product/B002PACRHC/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
https://smile.amazon.com/gp/product/B000WZ4KO0/ref=ppx_yo_dt_b_search_asin_image?ie=UTF8&psc=1
Hope those links work. For both of those the best bait is a half Goldfish cracker (peanut butter too messy for these).
Just PM me if you are interested.