Can't remember what reins you had, are they plain ones? I'm sure they are.
Go from the middle buckle - where the reins join. Unbuckle the reins so they are seperate.
Grease them.
Bung the rubber martingale stoppers in hot water. Heating the rubber makes it much easier!
Put a piece of baling twine through the rubber stopper, quite long, and tie it so it's a loop.
Put another piece of string through the stopper, exactly the same, tie it in a loop.
You now have a martingale stopper with 2 loops of string on.
Hook one loop over a fence post (or your foot!)
Take the other loop and pull (wrap something round your hand to stop the string cutting in!) and hey-presto, you are now stretching the hole in the martingale stopper.
You might need to do some wiggling to get the rein started, but with your free hand (hey, you can have 2 free hands if you can rope a child in to do some pulling!

) you can shove the rein through the hole and wiggle it along the rein into position.
Then do it all over again!
Obviously the 'hole/point end' rein is easier than the 'buckle' end rein.
The buckle rein requires some fiddling, but once the buckle and loop are through the martingale stopper, you should be able to wriggle the stopper down the rein.
Good luck!
ETA..... CROSS POSTED WITH HEATHER A - great minds think alike!
Greasing the reins helps a lot too!!