Depending on how you want it weathered you can;
- Really water down acrylic paint, paint it on a section and then dab it off with a rough cloth, leaving behind ugly splotches and marks. Drybrush light touches on all the edges. Do this with a couple colours, darker ones more prevalent on the bottom, lighter ones more prevalent towards the tops of the walls to get a nice dirty gradient.
- Stain it with tea.
- Literally throw clods of dirt and mud at it to try and stain the paper, drybrush some highlights.
Regardless try all these out on scraps first before trying them on your beautiful model.