How are doors aligned properly?

A belt will definitely help, and if you ever use encouragement, whipping yourself until you bleed, could definitely boost your confidence

if you are referring to the hinge pins, unless you've damaged the hinge parts, they simply slip back in. start at the top. tap 'em in with a hammer. aligning the hinges when they've been off the door can be tricky. i use a plumb bob to make sure the parts that attach for the door frame are perfectly in alignment. Then, put only one screw into each of the hinges on the door, but not tightened down hard. attach the door via the hinges and tighten the hinges to the door using the known to be perfectly vertical hinges to guild the hinge plates on the door to where they need to be

Doors are mounted in the frame to work properly and are level. When you remove and replace them place something under them to bring them to level with the hinges before re-attaching.

These are doors on hinges?  Then the hinge plates dictate that they only be screwed back on exactly the same as when they were taken off.  The plate is separated from the hinge by taking out the hinge pin.  Now you got a hinge plate in hand ..screw that onto the frame using the same holes. Top and bottom.  The other half of the hinge is already screwed to the door.  The only thing you are holding are the 2 hinge pins.   Now lift the door hinge and slip it into the frame hinge  and get the bottom one roughtly aligned too. Then insert the hinge pins. The door is now connected and swings on the hinges.  The striker plate near the door knob is the last thing to go on and it should go back into its holes. as it is made of metal.  Only done this job in every house I have worked in so 2000+ homes. Not one is different.  It has been around since "Fred & Barney" so, a long time.  Check You Tube if you be still confused. Pictures are understood better than words.

You must have the correct door for the correct frame they will have been fitted and should be easy to install as the screws will go back where they were in the frame, so pick a frame and just try each door till one fits perfectly the correct way , you open the hinges and with the door open you standing in the door entrance you slide the hinges into their slots while holding the door , you will need packing under the door to lift it off the floor or it will drag test and see first how much makes the hinges fit perfectly , then you place the door on the packing slide the hinges in their slots and thread a screw in the top hinge with your fingers as far as you can then the bottom you can then loose the door and use a screw driver , practice a few times till you attempt it for real screws in pocket ready good luck 

That sounds like bi-fold doors, and you are not alone in being confused on how to put them in. The secret is that prong that sticks up on top is spring loaded so you can push it down to get it into the hole in the carriage. The other secret is you have to adjust the location of the holes on top so that the door hangs straight and opens correctly. It normally takes several attempts and a couple of arguments to get one in. And it is best done from inside the closet, so that means you have to remove stuff to get in the closet to make the adjustment. And the mechanisms are poor, so they will eventually slip and you will have this problem every few years. You might consider replacing the doors with regular doors. It is easier over the longer term.

Put in one screw in each of the top and bottom hinges to place the door. Look at the alignment. Then adjust the alignment by deepening the slot in the wood with a chisel if it is too tight, or putting a spacer of thin wood, or just wood filler to push it out. When it is aligned, put in the other screws to hold it in place. You might also have to sand the edge of the door to fit.