With the reference image above. I want to make the bottom part which is twisted. At the top it go upward straight and the bottom part it goes narrow. I tried with the screw modifier but how can i make the top and bottom part. I tired to apply the modifier and extruded but the center point did not works. Any suggestion or help how it works.
You can't create the top and bottom without applying the Screw modifier. If you choose to apply, before applying lower down the Resolution of your curve and the Steps of your modifier in order to keep a low-poly topology. Then you need to model the top and bottom (with some extrusions).
If you want to work with a curve as much as possible you could create a spiral curve (you need to enable the Add Curve: Extra Objects addon, then ShiftA > Curve > Curve Spirals > Archemedian), play with the parameters in the Operator Box (don't create too many Steps if you want to stay low poly):
Then header menu > Curve > Set Spline Type > Bezier:
Then press V to change handle type and choose Automatic:
Bevel the curve in the Object Data panel > Geometry, keep Round selected and increase the Depth value:
At last, extrude the top vertex and the bottom vertex, and in order to reduce the bottom vertex radius, press AltS or type the radius in the N panel:
