First, you'll need to load your file into the editor. Your last played one should already be in the text box at the top.
If not, hit the dropdown arrow and select the one you want to edit. Now hit the Load button.
Now the options on the left should expand a bit. You want "Character Edit".
In the Character box, there'll be another dropdown arrow so you can pick the one you want to edit.
Expand the "Traits" section and just pick the ones you want to add, and yes, you can select Trait Levels too.
(Some people have issues with being unable to get "Slender" to work, though)
Once you're done editing your character, hit Apply (if there are any red entries, jiggle the mouse up and down until they go away) then Save. As long as the Editor says "Save Succeeded" then it's good to go.
Don't select contradicting traits (such as two different body types) as the game will crash faster than usual.
Load back into the game and load the edited save. As long as everything happened correctly, it should at the very least load into the game.