Want Password on My Passport

I have an iMac desktop running Leopard 10.5.8.   I’ve been using my 1T. " My Passport" as a back-up for my hardrive for a couple of months but it is NOT password protected.   How do Install password protection on this device?  Thanks in advance to anyone who has the answer.

Depending of the model drive you can use different applications

if the newer passport you will use this


but if you have the older passport: