This method is easy and cheap, yet results a very pleasent result.
Step 1: The transfers
Also you will have to buy some 'T-Shirt Transfers' sheets sold in many stores, including your local printer shop. Often the 'T-Shirt Transfers' sheets will only allow you to use them on white T-Shirts, but it is possible to buy them for darker colors.
I found these at 3-4$ per sheet, but I live in a country where everything is expensive, so you might be ble to find them at a much lower cost.
Step 2: Customize!
All you need to do at this point is to flip the text or image horizontal in an image editing software. If you don't hve such, the instructions on the T-Shirts Transfers will often guide you to a website where you will be able to download their free software.
In able to save on the number of sheets used per image, you can insert all your desired images/text into a word document or the like.
After this is done, go ahead and print it! Make sure to print on the blank side of the sheet. Of course, if you want colored images, you will need a color printer ;-)