Quick answer, it’s not hard if you’re handy. Anyone who has experience with drywall and a basic understanding of how a stud wall is built can put a tank into their wall. The only other skill that’s helpful is knowledge of electrical wiring.
First you’re going to cut a hole. You need to know if that’s a load bearing wall or not. If it is you will have to brace it in such a fashion that you transfer the load around the tank. Next you going to finish the stud work and construct a nice flat base for the tank to sit on.
A GFI electrical outlet should be placed in the top corner at this point. If you do not know how to do this you might need to call a friend who does. Then the tank goes in the space, patch up your drywall and trim it out with some wood. Not a hard job if you know what you’re doing.
More detailed instructions: http://www.walltanks.com/picture_library/instruct_jumbo.pdf