Plumb bobs are available in different sizes and weights. You can buy the one weighing between 4 and 10 oz for framing a wall at home. Normally, the string comes along the plumb bob but if it doesn’t, you can use any un-stretchable piece of string, like cotton, to hang the bob.
You have to cut the piece of string, measuring length from the ceiling to the floor. Fasten the string to the loop or hole present at the head of the plumb bob and it will be ready for use.
Now insert a nail to the ceiling, keeping a distance of an inch or two from the outside corner of the wall frame. Tie the other end of the string to the nail on the ceiling. Since the plumb bob is heavy, you would see the string is stretched to a perfect straight line from the ceiling to the floor. You have to make the string as the line of reference and make adjustments to the wall according to this straight line.
Now you have to transfer the ceiling points exactly vertical to the floor. You can do it by nailing the bob string to the upper point. Wait until the plumb bob automatically comes to rest.
You can determine the bottom point hanging right over the aligned post and use a pencil to mark it for further steps.
For aligning a doorway, you can drive a nail in the wall just above the centre of the frame and hang the plumb bob to it, falling vertically down the middle.
Take a measuring tape and check the distance between the string and the frame from the top and the bottom. These measurements must be equal and if not, it means the alignment is not perfect and you have to make the necessary adjustments to rectify it.