When we made t-shirts last year, we just had the graphics program do the mirror image, most programs like Adobe will have that feature. For ironing, I just put my teflon baking sheet on the ironing board and under the t-shirt and it worked very well. I bought my transfers at Office Depot and they worked fine. The trick is in when to pull off the transfer, some transfers needed to cool a bit and others needed to be hot, make sure you read the instructions carefully.
You also need to make sure that if you are using colored t-shirts that you buy the transfers for colored t-shirts since most transfers have a white background, not clear. Clip very close to the image but leave about 1/4 inch around.