Start with CSES Increasing Array II. It's the cleanest Slope Trick problem. Once you solve it, try the Sonya problem with the transform trick. Then Solve Buy Low Sell High. It's less obvious that Slope Trick applies. Understanding why the double-push works here builds intuition.
After these three, search for problems with absolute value costs on Codeforces. If you can recognize the pattern, you're ready.