I've been with my partner for 4 years I have 2 chidren from a previous relationship my children don't have any contact with there real dad he doesn't pay csa and they haven't seen him for 3 years.could my new partnes adopt them and how?

You hire an attorney, he contacts the bio father to have them sign over their rights to allow the adoption

It often works a lot easier if he had been paying support, since it stops, so if he does not want to sign, you may need to consider trying to enforece the child support agreement