usually I change strings one at a time

I slacken the machine heads lots and then the other end of the string is normally jammed in to the body by a tapererd wooden peg ( slotted) - sometimes these pegs are very tight and pliers have to be used, If you have to use pliers be sure to cover the peg with a cloth - so that the jaws of the pliers do not chew up the Peg - replacing the string is the reverse procedure.
Be sure to wind a couple of inches of the new string on to the machine head.
The machine heads on the right of the head should wind on clockwise and the machine heads on the left should wind on anti - clockwise - otherwise they won't line up with the nut or the strings will foul up against other machine head shafts