To mark the entries in the Royal Library Database, click on the small needles to the right of the entry. Once you have selected the items you want, click on the needle at the top right
In My Favorites, select the materials you want sent to Mendeley. and then click on the three dots that mean Export, Print, Mail
Choose RIS(Endnote, Mendeley)
Choose Download
Once the file has been downloaded, drag the file into Mendeley depending on which browser you are using - see under Import / Browsers if necessary