No one may take a (EASA) Skills Test without having first completed
all required course elements and having a Course Completion Certificate from their school (which the Examiner will want to see) to prove it. For PPL that means you must have been trained on all parts of the PPL syllabus, must have met all minimum flight time requirements (45 total, 25 Dual, 10 PIC, etc) and have passed all written exams.
For the Skills Test itself, remember that the test is for you to demonstrate your ability to act as PIC on a private VFR flight whilst carrying a passenger.
So don't be shy: Act as a Captain and treat your Examiner as a passenger and
tell them what you want them to do: ie Do you know how to fasten this seat belt? Let me show you. Please keep it fastened throughout the flight. The emergency exists are the doors, this is how you open them, please don't unless I tell you to.
Extra emphasis on Lookout and Checks - Just so the Examiner is sure you are doing them.
As OhNoCB says no one expects perfection and everyone makes some mistakes. Just try and relax and manage the flight (including any mistakes or perceived mistakes) as you would any other.
One thing I was advised to do when I did my skills test was, when talking to ATC, to start each initial contact with: "G-XXXX, Student pilot on skills test". I do think it helped.
I would not recommend this.
If you do I would expect an Examiner to tell you to stop and include it in their debrief. "G-XXXX Student Pilot" is reserved for use by solo Students. You are not solo and, if you pass, you will be recording PICUS (not DUAL) in your Log Book. Therefore it is incorrect RT and technically you could be failed on that one item (though it would be a monstrous Examiner who did so - and Examiners are not monstrous, just more experienced pilots than you).
Good Luck and, above all, enjoy.
PS: Remember one Section of the Test will be for the SEP Rating which is Oral questioning.
Know the aircraft you will be taking the test in (Learn the POH), this also applies to some of the planning.
Q: You have planned for a 2 hour flight. How much fuel do we need to carry?
A: The aircraft burns 25 Litres per Hour so to allow for Taxi and Contingencies we should carry a minimum of 75 Litres.
Q: Good. Why are you using the figure of 25 Litres/hour.
A: Because that is what my Instructor has told me to use.
Your Instructor is not on Test. You are!
Know why things are done, not just that they should be done.