Originally Posted by
budgetingjew
I don't even know how to change terminals in LHR landside...

Follow signs to the Heathrow Express. Do not buy a ticket as there is no charge for transfers between T5 and T123. Take the lifts down to the Heathrow Express platform.
Hope your bags have wheels.
Originally Posted by
budgetingjew
Just checked in in AMS for AMS-LHR. Connecting to LHR-FRA-SFO on LH.
Strange routing flying through the UK to get to FRA. It is for some cheap LH ticket? In which case, you should have done some home on here first. BMI would have interlined your bags and not required a change of Terminals. BMI is *Alliance. That is the key advantage of sticking to one alliance for one journey.