to do a T-shirt it is usually best to use a printing or publishing program rather than a word processing or designing program. A lot of printers come with printing programs. I know my HP did, and so did my Lexmark. Look on that disk first. You might have a printing or publishing program that came with your computer as well.
Make sure you do a print preview on the screen to rule out any obvious errors (like margin problems). Then print on plain paper using the same settings you would use on the transfer (except you can use draft and black and white if you prefer). Take the paper over to your shirt and make sure the size is going to work for the area you want to cover. Then, go back to your computer and send your image to print on the transfer. Allow your transfer to dry for way longer than you think you should, like 15 minutes or more.
Then with the Avery transfers, iron way longer than it says, like double. Make sure there is no water in your iron for steam, even if steam is turned off. This can really wreak havok on your transfer. I can't remember if the Avery were cool peel or regular hot peel, but if it's cool peel make sure you let the transfer cool before you lift it. If it's hot peel, make sure you don't let it cool first or it won't work. When you are peeling, start with a small section of one corner and slowly peel toward the opposite corner. If you start start to peel it and it doesn't peel really easily and cleanly, then it needs to be heated up and re-ironed.
After you let the transfer sit 24 hours I would wash them in a vinegar wash to set the ink. This is about a cup or a cup and a half of white vinegar in the washer, no detergent, rinse twice, then put through dryer. If you only have a few shirts in there, put a towel in the dryer with the shirts so they do not get bunched up.
Hope this makes sense.