There are online resources and books to help you teach yourself to embellish painted canvases.
I am drafting a list of books that are particularly helpful when embellishing painted canvases. Expect a tab with that header shortly.
SharonG has a wonderful article on her philosophy about stitching painted canvases on her blog. It's full of great advice.
Needlepoint for Fun has a general article about how to embellish on their website which you might find helpful.
I have a few general tips of my own—
Always include some tent stitches in a design if possible. This helps calm the design and gives the eye a rest from the fancy stuff.
Stitch the background first. It helps keep the background in the background and you can line up rows of stitching on either side of main areas better if you don't have stitching already there. Folks love using skip tent, basketweave or Nobuku as a background stitch so these are good choices if you are stuck. I personally go with fancier stitches than these with a lighter coverage than usual for backgrounds, but that's my own personal style. Do what seems most natural to you and you can't go wrong.
I also like to scale my background stitch so that it is a little smaller than the main motif on the canvas. If you have a figure on a plain background, find a stitch the size or a bit smaller than the head. If you have a bunch of flowers, pick a stitch that is around the size of the flower centers.
Remember light colors seem to come forward and dark colors recede. You can use this to emphasize and minimize parts of a canvas. Of course sometimes your canvas is a dark red flower on a light blue background, for example. In that case, you can do things like use a medium blue thread for the background and a red metallic or beads or silk ribbons for the flower to minimize the background and emphasize the flowers more. For every rule in the embellishment world, there are many exceptions!
Repeat stitch families. If you used a boxy stitch like mosaic in the background, use cashmere blocks or something similar in the focal points. If you used round stitches in the background, add a round stitch elsewhere. This helps unify the canvas. Or you can repeat a motif. For example, if your canvas is of hearts and you have a plain large background, choose a heart stitch.
Emphasize the directional flow of the design by choosing stitches that slant the same way. Of course not all canvases have things that slant left or right. If your canvas does not, use a vertical or horizontal stitch.
Take photos of your canvas as you work. The camera will see problems and mistakes that you do not.
Always make a color copy of your canvas before you start stitching. It often will come in handy later when it comes to positioning things on top of areas you have already covered in stitches.
Be careful about dimensional items such as ribbons, charms, beads and turkey work. All the areas of your canvas need to be in scale unless you are stitching a portrait of Dolly Parton.
Don't be afraid of ruining an expensive canvas. Trust me, you can't. Even if you rip out and accidentally cut a canvas thread or threads, it can be repaired. So just relax and play around. You'll be amazed at what you can achieve!
To read my other articles on Learning to Embellish a Painted Canvas, click on any of these links or use the tabs at the top of the page.