Beginner sewing onesie Sewing Machines Parts and Sales in Los Angeles

Tl;Dr Below

So I usually play games in my spare time but my new place we have been struggling to get Internet. I have taken on sewing as a hobby and made a pillow to start with with a zipper after watching numerous videos. I had some mistakes my grandpa has his upolstry shop on the same property I live on and I asked him what I did wrong and I did everything right except my tension wasn’t set right. I am now wanting to move on to making my daughter a onesie. There are patterns online but how do I print them to the size of my daughter ? She is 3-6 month cloths and is 5 months atm. I am new to this but think I have a natural talent or its an easy skill because I love doing it and I feel I do a good job .

Tl;dr I want to make a onesie for my daughter who is 5 months old wearing 3-6 mo cloths. How do I print the pattern to her size ?