One method is the "freehand method", whereby the forger, after careful practice, replicates the signature by freehand.
Although a difficult method to perfect, this often produces the most convincing results.
[1] In the "trace-over method", the sheet of paper containing the genuine signature is placed on top of the paper where the forgery is required.
The signature is traced over, appearing as a faint indentation on the sheet of paper underneath.
[1] A number of characteristics can suggest to an examiner that a signature has been forged, mostly stemming from the forger focusing on accuracy rather than fluency.