To make an online reservation you have to visit the B-Ticket website, after that you can select the Museum, the date and time of your visit, if you are eligible to a reduction you should open the tab ‘Reduced and combos’ choosing the correct fee.
The online reservation of a ticket grants priority of access at the date and time selected, furthermore the reservation fee of 3 or 4€ (depending on the selected cultural institution) is already included in the price of the individual tickets.
Before making the payment, you must log in your account, if you don't have one you will need to create it filling in the registration form (entering the data required), e-mail and creating a password.