Above all else, the Rams need to tackle well. The first man can't miss. And everyone else needs to be swarming the football to gang tackle opposing players if a tackle is missed (or hits but just can't succeed in bringing down tough runners like Deebo and Kittle individually).
The 49ers offense is pretty much all YAC. Shut that down and they're toast. Even more so than usual with a gimpy Jimmy G and left tackle (Williams). Assuming the latter even plays, which is really up in the air.
Easier said than done, of course.
Particularly as the Rams are not the surest tacklers...
EDIT - I would double Deebo but that leaves Kittle as a problem. The Rams need to get bigger in the secondary. Other than Ramsey, now that Fuller is out, everyone is undersized. Williams, Long Jr., Deayon... even the safeties are small in Rapp, Scott, and Weddle... that's 6 players... I don't think even one of them is 6'0"? I don't think so. A few are 5'10" or less. Could cause problems against a team like the 49ers. It sure would be nice to have a bit more balance in size.