It is advisable to have changed your passport prior to doing this, as you can use this as proof of your new name when applying for a driving licence.
To apply in N.I. you can access this on-line by clicking here.
You can choose to either send your documents by post or take them to your local DVA office.
(Note that to change your gender on a UK driving licence it is not necessary to have started hormone treatment or to have had any surgery. Additionally, changing your gender on a UK driving licence does NOT change your legal gender; this is tied to your birth certificate).