Add a new TileMap node

In the Inspector Tab

Add Create a new TileSet

Now you have the TileSet Window at the bottom of the editor

Drag and drop the tiles_packed.png to the Tiles tab at the bottom of the Editor

If you get a pop-up window

Click Yes / Accept

Change the Tile Size to 18 x 18

Also change the Texture Resolution to 18 x 18

Save the TileSet

So we can use it again

In different scenes

It now appears as a resource

in the FileSystem