Try to avoid things that will be especially challenging. If you really struggle with colour mixing or you immediately get stuck with an early painting it's probably because there's something really challenging about it, such as the colours can't be reached with the 5 standard colours. For example green leaves or a bright sky. Mark really doesn't let on just how many colours are unreachable with the basic palette.
Also avoid using overexposed photographs.
Starting with an apple and cup on a brown table or whatever... Something like that can be good. It only takes a couple paintings to really figure out the basics.. And then nothing will be too challenging because you will know how to mix every colour and have some confidence.
What you've done with the background reminds me of the leafy part of my first dmp painting. I regret doing that instead of painting more detail. It looks okay in the darker bottom areas around the nice leaves you painted, but the area above the rose could be done much better. Something I've learned is that creating some sort of abstract foliage like that is actually quite challenging. There was a tiny bit of it in that rocks painting I just did and it's not easy to pull off at all.
If I had to paint this rose photo, I would think that the blurry background would be the most difficult part by far.