The approaching club should submit a 7-day notice of approach to the served club to make it clear that they would like to complete a transfer for their player.
This is not completed from within The FA's player registration frameworks. This can be done through an email or an instant message but proof of the approach should be retained in the event that a league or county FA need to investigate the grounds of the approach.
The served club can end the 7-day period if they agree to 'waive' the notice period. This means that, rather than waiting for the full 7 days to pass, the player's current club can allow an approach to be made immediately.