I folded the top (wide end) of the bag inside-out - about an 3 cm (one inch). Then I used a regular kitchen knife to "smear" the PB into the bag. A bit of jiggling it and squeezing (squeezing it down like toothpaste) pushed it down to the pointey end. Then unfold the top and seal.

It was quite easy once I got going.

- You can buy pre-packaged packets of everything, but :
doing it yourself, controlling the quantity and
quality is well worth it, especially if you already have the machine.

My machine was just a cheapie from a discount supermarket - nothing special.