100% use solid doors - I’ve never tried to cut a hollow color veneer door, but I broke one once and I have a hard time picturing it doing anything other than splintering when the skillsaw’s blade hits it. Best case, if the cut goes through fine, I’d still want to cut a strip of wood to glue into the now-open end of the door for structural integrity.

If you go with solid doors, ideally actual wood, it should be as simple as cutting any other wood sheet. If the doors are plain, I’d cut the full amount from the bottom, if they have panels, windows, or other landmarks, you might want to remove material from the top and bottom so it doesn’t look disproportionate.

(I’d start with the longer door -absolute worst case, you can cut it down to use for the smaller doorway)

I’d get some stain that matches the color of the door, and make sure I have some sandpaper/sanding disks too.

Measure from the top, mark the line, measure it again, stand it up by the doorway and check it’ll fit and have enough clearance to move freely. Use a skillsaw to cut it to length (err on the side of too long - you can always remove more later), check the fit, if the cut is wavy, sand down any high points. Use the sandpaper to remove any splintery bits, and use the wood stain to color the fresh cut wood and any scratches/damage near the edge. If it ends up too short you can always cut a strip of wood, stain it to match, and screw it to the shortened end (predrill your holes to reduce the chance you split the wood). Or add a strip to the doorway.

For hanging the doors, I have a few questions: have your builders already put up full door frames, like with casings and jams?

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If so, you could take those off and see if there’s any additional space to work with (and check how square and level everything is).

I’ve hung doors but just old seconhand ones in a camp, and ones we made on a workshop and shed. We usually attached the hinges to the door, set it down on a wood shim so it sat where it was supposed to once it was mounted, checked it was level and plum, then marked the hinge side jam, cut space for the hinges, and attached it to the frame. For what it’s worth there’s much better advice for this part online.

The good news is that these doors aren’t terribly high-stakes. They’re interior doors, so you’re probably not worried about weatherproofing, or making them form a good seal when they’re closed or about water or anything like that. You just need them to work and not bother you. I didn’t ask about your overall comfort with diy tasks, but I think this is fairly approachable with a couple YouTube tutorials and the right tools.

My last bit of advice is on making sure the door is real wood rather than particleboard with wood veneer. Wood doors will show endgrain somewhere, usually top and bottom, even if it’s made from different sections and panels. If every side is similar grain and there’s a bit of a seam at the edges, I can’t vouch for what’s actually inside the door.