Tile Based Map Editor - Organize Layout

I'm trying to program a tile-based map editor and most of it is going quite smoothly except trying to organize the layout. Originally I was going to use 2 frames, one for the map, and 1 for the tile set, but after reading about frames I learned that that is bad practice and is also inconvenient because there is no way to have both frames in focus at once (so you need to click an extra time to gain focus when switching windows). So what I'd like to do is create a single frame application that holds 3 panels. One for the map, one for the tile set, and one for tile settings. This is basically what it would look like:

Note that the tile settings panel uses GUI elements (a border, a JLabel (which starts out empty), a JComboBox, and a JTextField) while the other 2 panels are just drawing panels*

Now I have tried multiple things which either did not work visually (panels were inside panels) or did not compile. One thing I tried was using BoxLayout to put the 2 tile panels vertically within a temporary panel which I then tried to add into the frame after the map panel (with FlowLayout), but that made it look like this: I honestly don't know what else I did that I should write here because in retrospect many of the tings I tried were silly or I don't remember exactly what I did. What would be the best layout to doing what I want?