Just follow the onscreen prompts.
It takes about 20 minutes
I did not expect it to be so quick. There is a link on the process page for DVLA use it and if you have your retention code, its done and dusted. You print off a page from DVLA and have 3 days to fit the new plates.
Better to check your motability account to make sure the changes have went through. It takes 24-48 hours to update the new Reg.
Similarly with Direct Line and the MYRAC app as well.