Within Ukelele goto File -> New -> Copy of other keyboard layout -> (Within Finder) Macintosh HD/[Ukelele install dir]/System Keyboards/Roman/French.keylayout
Now you should see a basic layout of the keyboard. Pressing control keys such as alt and ctrl should show a different set of characters on the same keyboard. Note that the Windows key is the Apple key.
Open up the Character map (also found within Internation/Input Menu and activated via the same drop down as the keyboards in the top bar) and click and drag characters where you want them active on the keyboard. When finished with the layout, save the file, something with extension keylayout and put it within ~/Library/Keyboard Layouts. Logout and log back in and use the Input Menu to search for the keyboard you just named and highlight its checkbox. The keyboard should now be available from the top bar drop down menu. In some cases you may have to rename the layout file and logout/login for the changes to take effect.