Thanks for your question. I will try to help.
The quickest thing to do would be to instruct a solicitor to prepare the legal charge and application to land registry to register the charge securing the monies owed.
If you were minded to attempt to do it yourself, the Land Registry has a CH1 form which you can attempt to complete yourself to reflect the monies owed.
If there is currently a mortgage already on the property then you would have to contact the lender to see if there consent is required to register the second charge at land registry. If it is required (as is likely) then you would have to obtain their Consent To Second Charge in order to now register the new second charge.
Then you would have to execute CH1 and the make an application to land Registry to register it by submitting their Form AP1, together with the legal charge, consent to second charge and also an ID1 for you and probably an ID2 for the lender.
The ID1/ID2 are identification forms require by land registry to verify the identity of private persons making applications who are not conveyancers and you would have to attend a local solicitor with your ID1 form for 10 minutes in order to get them to confirm your identity on the ID1 form.
The charge would then likely be registered against the property and land registry would send you an updated title register for your property reflecting this.
My goal is to provide you with a good service. If you feel you have received anything less, please reply back. Please remember to leave positive feedback using the stars at the top of the page.