Because papier mache is gennerally abit rough, it is easy to antique.

Try this on a sample piece to get the effect you want before you do it on the real thing: paint the piece the colours you like. Let it dry - the longer the better - even a couple of weeks is good. Then mix a thin sepia or brownish paint and apply with a brush over the area you would like to antique. With a soft dry lint free cloth wipe the paint off the papier mache. It will stay in the grooves. Take care not to rub too much or you will damage the original colours - that is why it is best to have a very dry colours to start with. This takes a bit of testing to get the look you like.

Or you can try a light application of a creamy brown shoe polish. Apply and rub off. This method does add some sheen to the surface.

Hope this works for you!