Applying for an annulment is effectively the same as for a divorce; you will need to fill in the appropriate “Nullity Petition” forms and send these off to the court with the applicable fee (see divorce costs). Once the other party has responded to the nullity petition then you can apply for the decree nisi and 6 weeks later the decree absolute.
It is possible to do all this yourself, however as there are complexities to the whole process we would recommend seeking independent legal advice to help guide you through the whole journey, especially if children or complex financial issues may exist.