As ever hex editors are not magic. If someone has a guide using one there is a lot of work you were likely not seeing happening in the background.
In this case the standard thing to do is find the health value (standard cheat search thing of search, take some damage (maybe be poisoned), search for things that changed, take some damage, search....). Once you have the location in memory you can then set a breakpoint and take a potion. Part of that will involve behaviours at maximum health so you don't go over the limit. Once you have those behaviours you can start editing to exceed what the game normally takes as a maximum.
In some cases you might be able to force a max health value above what it wants, though other times max health is a function of level, stats or something so not as easy to change.