I am currently working on a new art journal, with 'aged' pages. I first stuck down old tissue-paper sewing patterns and the instruction sheets on each page, overlapping them and scrunching them up slightly, let them dry overnight, then painted over them with dark brown watercolours, so that they looked really dirty and kind of muddy. Again, let them dry for a while or use your heat gun, being careful that they don't buckle or catch fire!
When the pages are dry, paint over them with very slightly watered-down white acrylic paint, but don't cover the whole page. Leave gaps here and there and really spread the paint around. It gets caught up in all the wrinkles and creases of the tissue paper and looks really interesting. Leave it all to dry overnight and voila!...aged pages ready to work on.
It took me a week to stick in all the tissue paper and then add the paint layers, but it's so worth it. I also used ripped-up old book pages to decorate the outside of the journal and sealed the covers with Mod-Podge.