From the photographs it looks like she prepared the soil, then placed each pallet and covered it with potting soil. Another method is to place the pallet face down and cover the bottom and sides of the pallet with landscaping paper or gardening fabric. You staple the paper or fabric in place on the bottom and side pieces. Then when you flip the pallet over and place it in the garden any potting soil that you pour over the pallet will be contained by the landscaping paper or gardening fabric. This keeps the good soil contained and also deters weeds. Whatever method you use, the pallets have a wonderful organic look. You also end up with nice, straight rows.
Click on the How to Create a Pallet Garden link below to read the entire article.