Buying generic pre hung doors is easiest. As you don't have to work out the hinges etc. You can probably get them with hardware pre installed too at a bit of an extra cost. put the door and frame into the opening and then make a bit of frame work around them to join them solidly with the edges of the opening/roof/floor. Leave yourself a small margin around the door for spacers to align the door. cover the frame work in plaster board, go for gold with the joint compound and spread it reasonably wide (~10cm wide over the join and don't be afraid to do a bit of sanding, then go for the finer top coat at ~20cm wide and sand again, it makes it harder to see there is a bump. Going too narrow makes the bump more visually obvious). you may need to take some plaster board off around the edges of the existing openings to remove any plastic/metal reinforcement pieces on the corners for a better finish. The most important thing is to take your time getting the door in the correct spot before you screw it in properly (square/straight) so the door stays where you open it to instead of naturally wanting to swing open/closed. If you want pocket doors then that's a whole world of hurt to retrofit, just pay a pro to do those.