T-shirts are most often made of soft cotton that's prone to distort during wear and washing. As casual items they don't need the military style finish of formal shirts. knitwear also needs gentle handling and soft finishing.
There aren't many features on a t-shirt or on knitwear, often there won't even be a collar, so ironing is a fairly simple process. Follow the method below, missing out the collar if you don't have one.
Collar: As with shirts a collar is the first thing ironed on a t-shirt. They would normally be one piece and one layer, the type you can iron from one side to the other with no problems, but there are some which have a stiffening and two or more layers, meaning you have to iron from the outside in (see shirt ironing).
Sleeves: are next and will mostly be short, the difficulty being sorting the distortion that happens because the fabric is so soft.
Body: is the final step just as in a shirt, the reason being that you manipulate the garment more when ironing the rest so if you do the body first you will crease it doing the sleeves and collar
