How to fuse properly to promote officers?

There is no right or wrong way to fuse promotion materials. What you need instead is a constant stream of this items as you need a lot of it even for simply promoting one to 4-stars. As you'll see when you check "Officer" tab, and then "Materials", each time you fuse 2 to 3 of a lower material type, you'll only produce 1 of the next tier. So basically, the item needed to promote to the next star level exponentially increases as well. What you'll want to do is secure supply points in the special operations (Chinook) or else, see to it that you are always participating in Fanatic General Level 5 rallies with your corps - as you get better tiered materials from it (from legendary supply chests).