Hey Cosette! Thanks for reading, I'm glad it was helpful! How you prep the ground under the boxes depends on deep the boxes are. If they are going to be tall and deep with enough room for plant roots to grow down, you can just use weed barrier or black trash bags in the bottom - this is what I'd like to try. If the boxes aren't going to be that deep {which is more normal} your plants are going to need to grow down into the soil below. In that case, I recommend turning over the soil or tilling it to make sure it isn't too compacted. You could skip this step if the soil is already loose in your garden area, but if it's lawn or something similar, you'll need to break it up a bit.

All the research I've done this summer indicates that about two layers of cardboard, two to three layers of newspaper on top of that, and then about 3-4 inches of bark chips {or leaves or straw or something else really fibrous} on top of the newspaper creates a good weed barrier. You would just put your garden soil on top of those three layers and plant into it like normal. The cardboard and newspaper will break down slowly, killing any weeds that are trying to come up but also allowing your plants to get through if they need it. Does that make sense? If you have a heavy problem with field bindweed {morning glory} I've also heard that it gets through all of that in 2-3 season, which is why I'm just trying to avoid it. But I hope you don't have to deal with that! Let me know if you have further questions.