Need help decoding

I got pretty frustrated with this as well.

I removed the 3-4 fork from its race and inserted its pin into the side cover hole and "locked" it in with its interlock. I had the 1-2 fork still situated on its race. Then I position the side cover such that the 3-4 fork goes into its race and the 1-2 fork pin is starting into its hole. The sidecover is close to seated at this point. I used a slim wrench through the remaining opening to hold the 1-2 interlock lever "open" until the side cover can be pressed down enough to more fully engage the 1-2 pin, pull the slim wrench out, and push the side cover down. I'm sure there is a more correct/appropriate method and/or tool, but this worked for me.

Not sure about your Reverse detent spring, but mine was also part of the side cover assembly. Don't forget it.