For the the png just look up "full body pngs" there's lots and if you want you can put a celebrities name in like this  "Selena Gomez full body pngs"
flower petals:

(If you want just look up flower petals and there's all kinds of them)

Start off with Phonto:
Open a gray background or if you want colored that's fine. Make the text the Text a color that goes with the background. Add your title author and whatever else you want and make it somewhat like what I've made here. Then save.

Now open the cover you just made in superimpose:

2. Open the flower petals and make them to fit your cover merge. If you want to change the color of the petals open the color you want and go to transform make the filter hue merge. Then open your full body png. And place it where you want it merge and save.

Picsart:
Open your cover and go to draw then open your flower petals in a new tab. Place it on the cover where you'd like it.  Then open another new tab and add your png move it where you'd like them save your cover.

If you want add a filter! I went on pixlr clicked stylize and used silk but turned down the opacity!