Once you have woven onto at least half the length of your needles in a flat stick weave, the tension on the needles will be set and you can adjust the needles to weave your circular project.
To begin weaving a circular weave, pull the two needles on the edge of the flat weave towards each other and continue to weave around in a continuous loop rather than back and forth. It may be easier to weave on the needles in a circular pattern if you pinch the ends of the needles slightly to help them fan out at the top. Another method of holding the needles in position is to insert the blunt end of a pen or pencil into the cylindrically arranged needles, to hold them open as you work. Don't pinch too hard on the needles as you hold them or you may change the shape of your cylindrical weave.
You may find it easier to weave a circular pattern if you rotate the group of needles as you weave, rather than trying to hold the group in the same position at all times.