Can this transfer be done in both directions: RER to Metro and/or Metro to RER, please?
What is the physical process - does one use the ticket to leave the RER and then use it again to enter the Metro station: Insert RER ticket into the barrier gate, retrieve the ticket, walk to the Metro gate, insert ticket and retrieve it to retain until leaving the Metro network.
Yes, transfers within RER Zone 1 are allowed between Metro and RER, or RER and Metro.
When there are turnstiles/gates between the two lines, within the same station, you will insert the ticket in the exit turnstiles/gates (to allow you through), collecting the ticket on the opposite side of the barrier, then when arriving at the line to which you want to transfer, insert the same ticket to enter the turnstiles/gates and again collect it on the opposite side. This is common when going between RER/Metro & Metro/RER at large stations like Chatelet Les Halles / La Defense, etc.