It does take a while for them to get used to you and the more you stroke or hold them the better they will be with you. hand feed them their favourite treat whilst stroking them and they should soon enjoy it. Mine loved celery leaves, sadly they are no longer with us but were so funny.
Try not to touch their rear ends (they don't like it) - stick to stroking them gently behind the ears - and just go very very slowly and quietly to begin with. If they throw up their heads sharply, take that as a signal that they want to be put down/have a pee.
Agree with coansha about getting them to associate handling with treats.
I can remember ours nipping once or twice when they were very young and one of them has done so once as an adult (when accidentally provoked by dd) but apart from that they didn't tend to nip regularly.