Let's look at your sentence: "Draw a big desert tortoise digging into sand."
The sentence is almost correct but can be clearer with a small change. The phrase "digging into sand" sounds a little unusual. Instead, we usually say "digging in the sand."
So, the better sentence would be: "Draw a big desert tortoise digging in the sand."
This means you want to draw a big desert tortoise that is digging inside the sand, which makes more sense!