Hi Jones24, I went through this procedure many many years ago (and I do hope things have changed since then!), so I'm not sure if this info will help!
You need to contact Social Services and they will advise you. We waited until after our marriage, which turned out to be the best way because the Adoption procedure wasn't particularly easy. They wanted proof of our commitment to each other!
We were 'assessed' as to our suitability, very much in the same way as unrelated Adoptive parents are. The fact that my son was 'MY' son was irrelevant ... Bizarrely, we both had to adopt him!
We had to attend lots of interviews and counselling sessions to discuss our Parenting skills, personal differences etc. They even interviewed my 3.5 year old to ask how he felt.
It all took many months and culminated in a Visit to a Magistrates court, who formalised the Adoption! The Magistrates were really lovely made it into a meaningful family event!
I remember getting very stressed at one stage, and asking the Social Workers what would happen if she decided that I wasn't a suitable Parent!! The whole thing was bizarre.
I do hope things are easier now! Good luck!