Yup, slide it on from the tang end. Once up against the shoulders at the ricasso, solder it in place with low-temp silver bearing solder and the corresponding flux for the type you buy. I prefer TIX and TIX flux. TIX melts at 275°F and is one of the hardest soft solders. It only takes a small amount of solder to do it, so go easy on applying it.
When using soft solders the amount of heat needed is very low. 300-400°F is all you need. A heat gun or a very small butane torch is a good choice. If you overheat the joint and burn the flux, STOP, clean it all up, and start again. More heat will not solve the problem, it will only make it worse.
Most of us who solder on guards make a simple jig from a coil spring that will slide over the tang and held in place with a clamp. Slide the spring up against the guard and compress it just enough to hold the guard in place straight. Place a clamp on the tang to hold the other end of the spring in place. You can make fancier jigs that clamp on the tang and have with long screws that push against the guard if you want.
