The old transfer and sub people used Heat Tape. You can find it at places like Coastal Business Supplies. Look at the link above on the left.
Also, most new t-shirts have a crease on them anyways. If your transfers have a Straight Edge on top, use a ruler or level just under the collar, about 4 inches down. The bottom of the ruler or level, if you're holding it horizontally, should be 4 inches down.
Then with one of the score lines on the level or ruler, the line (vertical crease) on the t-shirt should match even though it's a short score.
I than make sure the t-shirt material hanging off the sides is equal all the way down.
After the t-shirt is straight and equal on both sides, put your tranfer on with the top edge against the Ruler or Level. If that is straight, and your t-shirt is straight you're good.
If you back up a little, you'll get a better view and will see if the transfer and t-shirt are straight.
I've done a zillion that way, and I first started with the T-SquareIt and it's worth every penny.
Just google T-square It and you'll find Lou's website. I believe it's
www.tSquareIt.com I think it's under $50 shipped, and he also has U logo It or something like that for logos.
Where do you live, maybe someone can come over and help. A good cup of coffee, and a cookie will get you a bunch of help. Lol.