Solution to this is really simple. It’s just a quick keyboard shortcut.
Go to your Blender program and go to Edit Mode
. Subdivide your mesh ( You can subdivide it a lot depending on how big do you want your circle to be, if we were going to talk about resolution [Low-Poly/High-Poly] ).
Once you’ve done that select minimum 4 faces around 1 vertex and delete those faces.
Here is the trick. Select all edges/vertices around the hole
you made. Press Alt + Shift + S
to scale it in a circle form like show on the picture above.
Finally extrude and press F to make a face ( to close the hole ).
You can make your circle in a more better form by subdividing again and selecting faces inside including the bottom face and scaling them in a circular shape by pressing Alt + Shift + S again.
Quick tutorial, but could be useful. If you have any questions, feel free to ask!