Natural heads tend to twist when they are unmounted due to different air humidity and temperature.
When you are ready to put the head on your instrument and see that it is twisted, moisten (not soak!) the head carefully with a kitchen sponge, then put it on a flat surface and push the drum shell in the rim from above.
Put your drum loosely back together and wait until the head is dry before putting any tension on it.
Let it dry slowly in the air (best overnight), don't use heaters, hairdryers and don't leave it in the sun as it will make the head brittle and prone to tear.