One method is to build a brake out of carabiners. The minimum equipment for this is three oval biners plus a locking biner, and the diagrams below show how to construct the system with only this many biners. However, this setup doesn't give much friction, especially with a thin rope, and normally people use at least one more oval, as described in the text below.
To construct the brake start with the usual large locking biner clipped to your belay loop, as you would for any rappel. The diagrams show a double-strand rappel.
Clip the two oval biners A and B, with their gates opposite and opposed, through the locking biner. Pull a bight through the ovals.
Insert your third oval, C, through the bight, with the spine oriented as shown.
Clip the load strands back through the sideways biner C.
Flip C around and over the tops of A and B, forming a crossbar with its spine. Make sure that the rope ended up passing over the spine of C, not its gate.
As noted previously, the setup shown in figure 4 doesn't really give much friction. If you have a fourth oval biner D, it's a good idea to add it next to C, so that they form a sort of double-thickness crossbar. Both C and D should have their spines touching the rope, not their gates. (They can be reversed, but not opposed.) Only A and B should be opposite and opposed.
Don't try to do without the locking biner. If you simply clip A and B to your belay loop, the rope will rub against your belay loop and possibly destroy it through intense frictional heating. It is OK, however, to replace the locking biner with a pair of nonlocking biners, opposite and opposed.
The standard braking position for this standard setup is to hold the brake strand down at your hip, just as you would with an ATC. If you raise the brake strand to a higher angle, it could press against the gates of C and D, possibly causing them to open.
There are various ways of increasing the friction. If you have a huge load to lower and lots of oval biners, you can build two of these brakes in series. You can rearrange the brake bars to send the rope through in an S shape. In comments, Steed suggests using a locking biner for C, in which case it becomes safe to brake with the brake strand raised.
A possible advantage of the carabiner brake over a Munter is that the Munter will tend to twist up your rope. A possible disadvantage is that you may not have enough oval biners.