Here are a couple of onesies I made for a new baby.  Not wanting to pay for a pattern :) I searched and found this pattern, which is a 6 month size.  I found a sleeve pattern and added the sleeves.

I think I have finally mastered putting snaps into knit fabric.  The problem I have found is that with the limp knit fabric, pulling on the snap when you try to open it not only is difficult to pull, but also weakens the fabric around the snap, even with interfacing underneath.  What I have found works is putting the snap in per usual, then taking a screwdriver of some type, or an object similar to that, and gently pushing the opening wider. I have found a needlenose pliers to work pretty well.  Here is a photo.

After gently pushing the opening a little try the snap again to see if it opens gently, but firmly.  If you've opened it a little too wide, just use the needlenose pliers to squeeze just a little all the way around to close it just a bit.  With a little practice you will be able to do it pretty fast and the snap will open without pulling on the fabric too much.