The UrW mechanics for slash-and-burn means you need to use some techniques that aren't realistic to do it because of the restriction on how quickly you'd have to convert the land.
My current protocol is to clear cut the future farming area during the winter and move the trunks/logs out of the way and then place branches in 9 tile long rows with one empty tile in between the rows in 9*9 patterns. I then set fire to one row and return to the start to process the soil (the first tile has burned out when the last one is done). Most days two such rows can be processed. Why 9*9? When harvesting you have to process tiles one tile away, which means you can reach 3 tiles in front of you, so I can just move along the center of a 3*9 stretch, and 9*9 is symmetric. It can be noted, though, that UrW village farming areas consist of individual plots scattered throughout the area, possibly where trees once stood.
If I were to do it in a somewhat realistic fashion, I'd cover the whole area with branches (not cutting any trees beforehand, at least not in the area, but possibly beyond to limit the spread of fire) and set fire to one end, letting the fire spread and then spend weeks processing the soil, but that's currently not possible with UrW's model.