The main problem with t-shirts is the fabric is easily distorted so you want to ensure the iron doesn't catch and add to the distortion. Make sure the material is flat and put very little pressure on the iron.
Collars
Most "polo" style t-shirts have collars which are made in a single layer. They often curve and so if you try to lay them flat they won't. Lift one end of the collar and place the iron on the other end. Guide the iron across the part you have flat. Move on a little bit further adjusting the collar as you iron only that part of the collar that falls flat each time.
Sleeves
The most important part of ironing any sleeve is setting it up. Start by lining up the bottom seam and then flatten the rest by moving your hands upwards over the sleeve. Lift the top of the sleeve if material is wrinkled underneath and roll it out lightly with your fingers.
Move the iron from the seam upward just using the weight of the iron. With the second hand slightly raise the sleeve and make sure the underneath has no creases and continue over the upper edge. Move to the next section along and repeat until that side is done. Flip the sleeve over, flatten the sleeve and iron the same way if necessary.
Now do the same with the other sleeve, making sure you have the shirt face up.
