If the rear derailleur breaks, you will need to create a single speed bike by removing the derailleur from the equation. To do this, break the chain, and wrap it directly from the middle chain ring to one of the easier to middle gears in the back. Choose the rear gear that brings the chain closest to meeting at a link, so that you can make the chain as tight as possible when you join it again. Now break the chain again to remove the excess links, and rejoin it. This is easier with the chain removed from the chain ring, but still looped around the bottom bracket/through the front derailleur. Place the shortened chain around the rear gear that you chose, and hook it over the top teeth of the chain ring. Now pedal the chain ring forwards to “hook” the chain onto the chain ring.
If you are lucky, you will have found a “magic gear” which gives you good chain tension. However, pedal carefully as you have no way of adjusting tension on the chain and so it may well bounce or slip from the gear you have it in. Do not try to shift the front derailleur.