Learning to ride lateral work is tough. I've been working at it for 6 years and my coach finally told me that I have a good feel for it.
It's honestly worth it to sit down and read some classical dressage books, and some bio mechanics books. It really helps you to understand what the horse is doing with his body in the laterals, and movement in general.
Personally, I would learn the travers first before the shoulder in. For this movement, you keep the inside bend and the shoulders, neck and head straight on the track, while moving the haunches in. You really have to utilize your outside leg and ride the outside of the horse to make this movement happen. You must keep the shoulders straight!
Once you have your outside aids working properly, while keeping an inside bend and the shoulder straight (this might take a while) then keep these same aids on for the shoulder in, except now you are keeping the haunches straight on the track and bringing the shoulders in. Shoulder in requires a lot of outside leg. Keep working and if your coach isn't helping you to learn, then try a new one.