Well, I started by drawing from imagination all I remembered of cactus (which was not a lot).

 

Then, did the study:

 

 

I drew from imagination:

 

 

And I actually didn't find something to fix. Cactus is really abstract and kinda flat, so after a good study I think I did a pretty good job. 

It's definitely not the best subject to study but that was what I wanted to draw at the time. I think I improved a little when come about cactus hahaha.