This is usually why people add hair, but if the hair is thick enough, there's really no need.
I braid my weaves starting out horizontally in opposite directions at the nape and connect the left end with the braid that will be my new braid going towards the "right". Then I start braiding them in a semi circle as I move closer to the middle of the head, connecting each other braid to the other semi-circle so I sorta of end up with a swirl, like a beehive but not quite. When I sew them together, I sew down, and it gives me a flat base, and then I sew the tracks horizontally, until I get a little past the ears, then I start sewing in a semi circle... and close up the top by making a tight circle, and then sewing the circle closed.
Imagine a circle it looking like this (-) and sewing the curves tight to the slit in the middle.
I hope this makes sense, when trying to explain how you do something... it sounds like a dag-on foreign language.