Choose a level and start playing. Each difficulty has its own puzzle and saved progress, so you can return to the challenge that suits you.
Start with Easy if you are new, then move up one level whenever you want more candidate work.
Each level opens its own playable page, and your browser saves that level separately.
On a keyboard, use the arrow keys to move between cells and N to toggle notes. Read the beginner guide or try a worked naked-single example.