123-reg.co.uk:

1. Login to your 123-reg account. (the login details are sent to your email when you register your domain name).

2. If there are any extra messages displayed, skip past it by clicking ‘GO TO THE MAIN WWW.123-REG.CO.UK CONTROL PANEL‘ to go to the main Control Panel.

3. Scroll down to the section labelled ‘Manage Domains’,  choose the domain you want to make changes to and click the ‘Modify Domains’ button.

4. Click on ‘Change Nameservers’

5. Scroll down the page and enter the nameserver 1 and nameserver 2 names provided by your hosting company in the respective boxes.

6. Click ‘Change Nameservers’.

This should be it. The change usually happens in less then 1 hour, although it can take upto 24 hours in certain cases.