It's surprisingly easy and I bet you have all the supplies you need on hand.
4-6 tea bags
4 cups water
4 sheets of paper with your printed message
First start with plain white printer paper and print the text you want. Then tear the paper into the shape you want. These were inserts so I tore them into small rectangles that would fit inside the envelope. After tearing them I crumpled the paper to give it a wrinkled look. You can skip this step if you don't want the paper to look as distressed. I tried it both ways and preferred the wrinkled look.
Next boil 4 cups of water in a small pan and add 4 to 6 tea bags. Make sure the tea is a dark leaf tea or you won't get a good stain. I wanted the stain to be dark so I added 6 teabags. Then I let the tea steep for about 20 minutes.
Then fully submerge the paper into the tea on the baking sheet. It's fine if the paper overlaps. You can leave it to soak for about 30 min to an hour.
I was so happy with how they turned out I stained the paper for the envelopes as well. I did eventually design my own tea stained background so I could reproduce these and sell them in my printables shop. But if I had to choose my favorite I liked the hand made version better. It just looked more authentic and vintagey to me.
Now wasn't that easy? You'll never need to buy age, vintage paper again. I hope you have a not too spooky and really fun Halloween.