How can I connect my WD Passport to my iMac for backup?

Here is the normal procedure I use.

• Connect the external disk using the vendor provided USB cable.
• Open Disk Utility and completely erase the disk
• Then reformat the disk using Disk Utility. Format it the same as your HD.
• Do not use the maintenance utility that came with the disk. These apps will almost always cause problems and should be deleted.
• Open Time Machine and carefully follow the instructions to schedule your backups.
• The backup disk should be dedicated only to backups and not for anything else.

You mentioned that you are out of storage. That means that you will also need another external disk on which to store files.

I done this on mac book air go to settings > time machine > click on + > then you will have the disk there > select it > if you have something there it will warn you if you want to delete everything and then it will start backuping

Which exact Mac do you have & which exact WD Passport model?