Preview is not a PDF Editor, so you can only add an annotation layer above the existing PDF.
For the character string "V 2 O" you select the Text tool from the markup toolbar (shift+cmd+A) and then just enter the text string. You have control over the text color, font, and font size from the Text Style tool. It would appear that you are attempting to draw freehand these letters using the sketch tool and that isn't much fun.
As has been pointed out, we need to know what your plan was here in your submitted image. Terms like "wonky" do not tell a story about your intended accomplishment. We are not clairvoyant…