When starting a new level you may want to disable lighting and select a grid size to make it easier to see without having to add lighting yet.

You can turn off Lighting with the shortcut 'Shift+F6' or use the Layers tab on the top toolbar to disable the rendering of the Lighting layer.

By clicking in the main level window of the editor you can press the '0' key to toggle the grid visibility on/off, then press '4' for the correct grid size used by Hammerwatch.

Select the 'Tilemap' tab on the right side of the editor and click on the name of a Tilemap you wish to use, check your brush settings at the bottom as this will determine how large an area you paint at a time.

Then just draw your tiles with the 'Left mouse button' and delete tiles with the 'Right mouse button', you will only delete tiles that are the same as the type you have currently selected.