Check out this post which includes CSS that moves completed to-dos to the end of the desktop note viewer.
For a cross-platform solution not requiring the note viewer, manually sort selected to-do lines if needed (with the instructions below), then move completed to-do lines to the bottom (with End and Enter).
If neither of the above work for you, check out different ways to manage your to-dos as described here (and elsewhere in the forum): Tasks as in Evernote