For purely "ageing" fabric (rather turning it beige or brown), I put six teabags in a teapot, fill with boiling water and let steep for 10 minutes. Then I pour the tea off into a bucket of water. I put my fabric in and swish it around a few times over the next 3-5 minutes. That's it. :)
Stronger tea (more bags, less water added, longer steeping, leaving the fabric in for longer) will give you beiges or browns. A friend of mine made a patchwork quilt entirely from fabrics stained different shades of tea.
No heat setting required, tea stains are permanent. I do wash my tea-stained fabric once before using it because a bit of the color tends to come out in the first wash.