To finish your knitted items, you will need to steam them. This relaxes the yarn and can help to straighten edges.
Always make sure you set the steam iron or steam bed for the material that you're using - you don't want to burn or melt it!
For the best result, pin your item to your ironing board in the exact shape you need it to be. Make sure the pins you use don't have plastic heads. Hover the iron over your piece while releasing steam. Be careful not to press on it, as this flattens the knitted structure.