Using a small blob of greenstuff rolled into a squat sausage, put one end on the model's head and gently press it down. Use a flat tool with a sharpe edge to alternately flattern the top and round the sides. Take care to get the bottom edge, where it sits on the head, to look straight. The shape will distort constantly, so have patience and keep gentley working it back towards the slightly conical (narrower at the top). Tease the greenstuff to a sharpe edge at the top by pulling it up and outward, then flatening the top. This takes some practice but it is a very useful gs skill to have for other projects, so keep at it!
Turbans are tricky.
Look at lot of photos online! There are a few different types, so pick one style and stick to it for each one you make.
The easiest one (if you can make fezes look good) is to make a fez, let that cure hard, then add a "doughnut" of fabric round the base - it's very easy to twist as sausage of gs into what looks like wound-up fabric.
For a non-fez bases turban take a big ball of greenstuff (bigger than the model's head!) and press it onto the model's head. Get the shape right before you move onto the next part. Look at those images on Google again! Once you are happy with the (quite complex to describe) shape you want to press a blade-style tool into the ball to make the edges of the fabric strips. I found this quite hard to get right! acdh strip has to go over or under it's neighbours and can't look like it just stops. The ends have to be either tucked under another strip or curve under the edge of the turban where it meets the wearer's head.
To finish either sort of turban it looks good if you can find a tiny decorative crest or gem to press into the front - centrally above the forehead. The High Elf plastics supplied most of mine but there are probably little bits in several ranges that would work.