This is how I would edit your writing:
Temakizushi is a type of sushi. First, you put rice on a sheet of seaweed the size of your palm. Next, you add any toppings you like such as tuna, salmon, avocado and so on. Finally, you wrap the toppings with the seaweed just like you would wrap a baby with a towel.
I personally find using firstly, secondly, thirdly - to sound awkward. I know it's technically correct English, but as a native English speaker who has spent a lot of time around business professionals, it's just rarely used. I think it's safer not to use them since they can cause a little confusion or cause the listener or reader to start thinking about the words you've chosen instead of just listening to or reading your story.
Also, I believe that "finally" sounds better than "lastly."
Good job! Keep up the good work :-)