Just make sure to choose which service you will be using(I always use first class international) , add the right amount on weight, plus 1 ounce (I always put everything in the envelope, before I print the label and the extra ounce is because the label it is going to be printed), once the label is printed, signed at the bottom and add the date.
An extra step, I always take a picture of the complete package with the label, and sent to the buyer through the conversations, so they can see the packet with the information and let them know that their order has already been shipped.