You first need to apply for a logbook loan, by either contacting a loans company on their website or calling them directly. Your application will generally be handled by a customer service advisor who will check if you meet the criteria. In most cases your car will need to be less than 10 years old, you will need to have a regular income and the car needs to be fully insured and have a valid MOT certificate.
Once you are approved for a loan you will then need to sign the paperwork. Companies do this differently, some will require you to visit them in their offices whereas others will come to meet you. The logbook loans company will keep hold of your V5 or logbook and you will still be able to drive your car as normal.
You need to make the regular payments on the loan otherwise your vehicle could be at risk of being repossessed.