CPL/IR preppin'
To be fair when I did the CPL it was all about being able to prepare a plog and make sure you had decent alternates and the like. Most all of it is what you've done before except you really need to be "professional" in the A/C.
Also it's vital to know the A/C you are flying, what kind flaps and ailerons as well as trim tabs does it have? What is the power source etc?
Some examiners will ask you questions on the A/C and so you have to be prepared for this event.
The IR is far harder and the best way to prepare for that is in my view grab a decent manual about IR flying. What you learned on the ATPL is one thing and what you actually do on the IR course is nay quite like the ATPL.
Know how to read a jeppesen plate from the beginning to the end and every obscure meaning as well. It's not too bad.
Know what a hold is, how to enter it etc.
Again if you purchase a half decent manual on that sort of flying it should be telling all about it.
Try simply googling "IFR holds" or something similiar.
You will also need to know your limits as a single pilot performing IFR flights etc.
Finally, when you do start the IR don't be afraid of asking how you should best prepare for an IR flight if it seems you always end up not knowing what's wrong with the plan.
That's all I can think of.