How do I install it?

You click subscribe in the workshop and it will download. You will need to restart Company of Heroes 2 though.

In regards with the OKW having different weapon models than osheer, you can use the strum officer type of squad. One with a pistol, buffed of course so it will scratch at least, and the rest with either rifles or with what they came. For example, having the volksgrenadier unit made of 4 riflemen and 1 officer, or you can use the sturm pioneer's weapon for the smg replacement. It doesnt have to be mp-40 , heck you can use the fg 42, it won't be historical accurate at all, but it will be a fun ride to have the volksgrenadiers with a fallscrimjager next to them.