Unless you have a liking for screw holes in your finished work the best option will be to glue a scrap of wood to one face of the blank.
Plane one face smooth, if the blank is square now would be a good time to cut it into a circle on the band saw and while you are there cut a disc of ply or scrap softwood about the same size of your faceplate. Now glue the two together with a sheet of paper between as this will make separating the two easier, clamp and leave to dry.
You can now screw the faceplate to the scrap wood and then mount the faceplate on the lathe having first selected the slowest speed. Bring up the rest parallel to the lathe axis and rotate the blank by hand to check it clears the rest. Take a sharp roughing or bowl gouge and turn the edge of the blank until it is circular, move the rest to the face and take a few cuts off until all the face is being cut.
You can now start to turn the shape of the outside with a bowl gouge, once finished turn the inside aiming for an equal wall thickness, don't go much below 10mm for a first effot.
Once you have turned your shape it can be sanded & finished before a sharp tap to separate from the backing and clean up the botton.
Worth checking the schedule on the woodwork channel as they often have turning programmes, its a lot easier if you can see it being done.
JasonB