I have the workshop manual but I cannot discern if this is something I can do myself. I'm 'fairly' good with things, can someone point to a good how/to video? I've seen some youtube vids on this but not for MY bike.. not sure if this is fairly universal.

This'll get the ball rolling. Here's the first video i landed on that's quite relevant to your 400.
I believe a lot pull them out like this guy.
Just be careful not to break the drill off or accidentaly pop it into the bearing behind.
I just dragged the video along on silent & he doesn't show fitting the new seals but personaly i just grease them & line them up by hand to get it started if you can, then find something to cover it fully & tap it in flush & square on.
Don't go beyond flush or not by much if you end up a bit further in.
ps, not sure if he shows how he undoes the crank nut but i just drop i thin-ish piece of alloy between crank & cluch gears to hold it still.
But you may need the clutch off too, so idealy need a holding tool for the hub or a good air wrench & could probably hold the center hub by hand with a rag & air wrench the nut loose.
See what others say as we all do things different.
I shouldn't teach bad habits but i just lever the seals out carefully without scratching surfaces, even the mag side i get under it & up behind without damage.
Just go with the flow--screw idea looks best.

www.google.com/search?q=Yamaha+dt400+cra...vid:IKHZgFcTVfU,st:0

Fantastic video thanks for bringing it our way. Nice clean work and excellent advice.

I'm somewhat certain the 77 has the "castle turret" bumps on the seal.  Bumps go towards the bearing...  you don't really want to go past flush as RT325 said...  bumps will keep you from pressing in too far and restricting oil flow to the bearing.

P.S. Use Yamaha OEM crank seals...  should still be available from Yamaha for the 77 (they were last time I checked).  Aftermarket crank seals are often not correct and can be of dubious quality.