Not sure I have ever seen a video of that, changing valve guide seals with heads in place
Its fairly easy to explain
Valve guide seals are inside the valve springs, so each spring needs to be removed to change that one seal, but if you take off the spring the valve is free to fall out, and into the engine, which would be a bad thing, lol
This is why they are usually only changed when the head is off.
So to change them with head in place you need a way to hold the valve up so it doesn't drop into the engine when its spring is removed.
There are three ways to do this
Put piston at TDC and use constant supply of compressed air via spark plug hole to hold valves closed, use wrench on crank shaft pulley so piston isn't push down by the air pressure.
I don't prefer this method because of two things, power outages(lose air pressure), lol, long shot yes, and the oops factor, if you happen to push down too hard on the valve stem you lose the air seal and OOPS the valve drops
Inflatable rubber bag that fits down spark plug hole, and then piston is moved to top and bag is inflated, air pressure prevents valves from dropping, tie off crank
Small diameter rope, the tangle free kind, you feed the rope down spark plug hole and it will coil up, move piston up and the coil of rope will hold the valves from dropping, same as above, tie off the wrench
You need a smaller valve spring compressor to get it into tight places to do this
And extra "keepers", keepers hold the valve springs to the valve stems and they like go flying or otherwise disappear so have intake to engine covered, don't need no metal pieces going down the carb, lol.
Better to have extra keepers on hand just in case
Replace seal then reinstall that valve spring, intake valve is the only one that matters but best to do both valves in each cylinder.
Then move to next cylinder
Takes a bit of time but no special knowledge is needed, just patience
This type of spring compressor will usually work for this:
https://images-na.ssl-images-amazon.com/images/I/61A82tKjcrL._SX466_.jpg