Like this
1) Build them a suitable structure out of rubble rock, build it
ON THE GLASS BOTTOM. Make sure the 'mouth' of the cave is facing the way you want it to or they may host the backside of the rock.
(The glass part is absolutely crucial, because if you just put rocks on the sand the shrimp might get crushed by digging up that same sand.)
2) Cover these rocks with sand, leave a little hole in the front of the cave to give the shrimp a starting point (also makes it more likely he'll build there).
3) Introduce them where you want them -- The idea is to build something that looks safe, attractive (to you AND the animals), and properly place.. Otherwise, if you just let them go they may build a nice little burrow on the backside of the tank facing the wall.
_______________ My personal suggestion _______________
4) Don't get bummed out if they don't do exactly what you want. It's YOUR job to make them a little area that's so attractive they have to build there.
I used to keep Tarantula's when I was younger. I had gotten one that was an absolutely prolific webber.. In his cage I build a cave out of large rocks, and placed dry sticks over the top of it. My goal was obvious, I wanted him to live in that cave and web it up.
Put him in, and he dashed to the cave. Cool. I'll see him tomorrow. The next day when I woke up, he built the most immaculate and impressive webbed cave I've ever seen. Dug out the floor and laid web, webbed up the walls of the rocks and the branches.. It was absolutely badass, and *exactly* what I wanted him to do.
Next morning when I woke up? He put dirt all over the ENTIRE damn thing. His awesome web fortress made of nice rocks I selected for shape and color looked like a mud pie.
I was less than pleased.. But he obviously thought his fortress could use some camouflage. Can't exactly be mad at a spider either.. So if you wake up to see your shrimp/fish had dug out the backside of your cave and are facing rock structure or something try not to be too bummed out
