This can be carried out using either spectrophotometry or by comparison of known standards on a gel. Problem with spectrophotometry is that proteins etc can affect the readings. This is carried out by running known standard concentrations of DNA alongside the DNA to be quantified.
Concentration is estimated by eye.
Use dilutions of a commercially prepared stock eg. Generally 100V is OK for most purposes. 25V if running maxi gel overnight. The buffer is re-used. Set up blot as described below in picture.