It depends what stone you have available. Most flint and chert can be knapped into shape pretty easily, but if there is none around you, that is a problem. And of course, you need the knapping skills.
If you want to grind one into shape, then as said below, find a cobble as close to final shape as possible. Grinding can be hellishly slow - have a try grinding flint - but other rocks, such as the greenstones, grind far more easily. And with this paricular stone, you can 'cheat' with an angle grinder to take it down to near finished size/shape, and then grind by hand on a sandtone/quartzite (an abrasive rock, whatever you can find) to out the final polish and cutting edge on.
I hoep that helps but be prepared to spend time on it
Cheers
Paul