Ok here is what I've always done over the almost decade that I've been doing this game! What you need to do is make a cave for your nem typically the depth needs to be about half the width of the nem fully open or 3-4" is a safe bet! The most important thing is to have the cave blocked off completely on the inside and about 1"-2" around depending on the size of the foot of the nem! Once you have your cave built gently massage your nem so he shrinks up as little as possible then place him in the back/bottom of the cave. Turn off all current until the nem starts to expand then turn back on the current.
Key notes........to keep him from wondering!
Make sure your nem will be in a place of variable current(not a blast zone)
Medium-moderate high light
Also if you have clowns this will help the nem to settle in quicker / prevent wondering,
You may have to move around corals to build your home for your new nem so don't put them back in that area until you see how your nem responds once you know he is settled in usually a couple of days to be safe you can place corals back In and around the nem!
Remember to allow room for growth
BTA12-14"
Sebae 12"-24"
Riteri 18" (no cave for this one)
With my cave method I've never had a nem pack up and move on me ( if you build their home sound enough)
Good luck to you and your nem!