Step 4: Seal it
I tried using double sided tape for this, hence the pictures of it behind my tea bags. It was hard to get it inside and then get the backing off then get the bag in place well.
Sellotape is much easier. You could use glue but it might not dry clear and it could poison your friend a little.
Cut a small slither of sellotape a little narrower than the slit in your tea bag. Insert on end of the slither into the tea bag facing up. Press down on the bag to get it firmly stuck then slide the side of the cut that the tape is on back, careful not to rip the cut wider. This should allow you to poke the tape into the other side of the cut. Once in place, press firmly.
At this point the slit is almost invisible if you've done it well. Most tea bags are a little crinkled anyway, screw your tea bag up to further hide your modifications, the extra creases will only help the slit blend in even better!