Plant flower beds where you can see them. Especially consider the views from private outdoor spaces such as patios, decks, and terraces. Don't forget about how the beds will look when viewed through windows from inside the house. Also consider how neighbors and passersby will see your gardens.
Use a garden hose to mark the edge of your bed before you create it. It's not a bad idea to leave it in place for a few days to give you time to work around it, view it from different angles while envisioning the plants, and adjusting the boundary. You can also mark the edge with flour, lime, or landscape paint. Pencil beds into your sketch.