I use a size 16 or 18 needle. My sewing repairman recommended a size 18 leather point needle to go through the plastic better. The tension needs to be loosened a bit on my machine. After talking to the repairman, he said that some newer machines aren't really set up to deal with heavier thread...
For those who want instructions- I make my bags minimalistic.
Start with a clean bag and take the bottom strip off. You can cut it close to the tape. I find the bags are often crooked at the bottom. Cut them straight.
Decide where you want the bottom of the side picture. Measure about 5 inches...