This is the most comprehensive and well explained guide I found on the internet about the string method.
Determining Wheel Alignment - String Your Car - Circle Track Magazine
In addition to that, you need to know how to move the alignment of the wheel. For that, you'll loosen the lock nut on the tie rod which usually takes a 22-24mm spanner.
This here is the lock nut on the tie rod, highlighted in the 'B' circle.
Once you've done that, you need to insert a 12-14mm spanner in the notch made at point 'C' and twist it to perform 'toe in' or 'toe out'. How you need to measure that, that can be seen in the above mentioned link.
Once you're done with the alignment, tighten the lock nuts on both the tie rods.
I would advise you to jack up the car and put blocks or tiles underneath the wheels and then drop the car onto the blocks or the tiles (I've used marble tiles). The car needs to have its weight down on the wheels to align. Do not jack the car up and then do the alignment as it'll be all over the place then.
Hope that helps