You'll find good resources on the web that have already been written/video'd with a search term like "How to Mortise Hinges on Interior Door".
If I wasn't using a purpose-built jig for this task, I'd use a router freehand to get most of the hinge mortise cut, and finish the edges with a sharp chisel.
It's not clear if the existing jambs are already mortised for hinges, but if they are, I'd make a story stick** to mark your door.
Pay attention to how plumb/square the jamb is, as that'll affect where the door will sit within. (Just as an example, if the head of the jamb is sloped, you'll either accept an uneven reveal across the top, or custom cut the top of the door to fit the opening. Either of these will affect where the hinges go.)
When you've cut your mortises, be prepared to play around with the fit... sometimes strategically loosening a few screws to get the hinge leaf to sit right will be all it takes to get a door to fit well.
I'm not going to lie to you: doing new doors in old construction isn't easy. Take your time. Do the least visible one first and learn from it.
** story stick (story pole) -- a stick (1x1 or 1x2 works well) around 78" for most interior door applications but ymmv. Simply hold it up to the top of the door jamb on the hinge side, mark about 1/8" for the reveal, then continue down the stick and mark where the existing hinges start and stop. ((This is where you need to know if the top of your door jamb is level! If it isn't, you should probably cut the door first, then lay out your hinges.)) Carry the stick over to your door; set it aligned to the 1/8" mark and the top of the door, and transfer the marks for your hinge locations. If you're lucky, all your doors are the same, and you'll only have to use one set of marks.