Fold in half or in three, as long as the edges line up it's ok. I often make another fold for the bottom, because it looks nonchalant. Then sew around thee sides, leaving the fourth open.
Time to decorate. Here I have used a stamp set (Djeco) that belongs to my daughters. Most op the time I use grey or black ink for the printed look of it.
Put the present or letter inside and sew the fourth side. Or close it with just a staple and a punched heart, like I did over here.