Does your sewing machine have a top-loading bobbin? Or a front-loading bobbin? I think that is what makes the difference.
If you have a top-loader, it's enough to cut the hole to fit the machine.
If you have a front-loading bobbin as I do, typically you make the hole larger than the machine and create a plexiglass insert to fill the gap between the hole and your machine. I believe hardwares stores will cut plexiglass for you to a paper pattern; you would add legs to the plexiglass to keep it at the correct height. Note: Some machines come with an extension table to create a flat surface. If you have one of these, you don't need to cut a new plexiglass insert. Alternatively, instead of creating your own insert, you can purchase a ready-made table extension for your machine and cut the hole to fit that table.
Edit: Here's a tutorial for the top-loading bobbin:
http://frommartawithlove.com/diy-ike...able-tutorial/