How to prepare for a screen test

This is a difficult question to answer due to so many different casting directors and directors wanting different things from a screen test, but I will go over a few major dos and donts for the first screen test. And also, how to practice what works for you.

My first tip would be to record yourself BEFORE your screen test so you can get an idea of camera frame sizes and how you come across on camera, try different frame sizes and don't be afraid to ask what frame they are using before you start your casting. It doesn't look bad, it looks like you understand the casting process and are experienced working with cameras.
Remember the casting director wants you to do well, they have called you in because from your CV and image you COULD play the role, this screen test is your chance to shine.

Once you have figured out the frame sizes and when you get into the casting a major no no is DONT do your reading directly into the camera, however sometimes the casting director may ask you for this. If in doubt, always ask where they would like you to direct your dialogue.
Some casting directors don't mind you asking, however some of the older generation find this infuriating, so always direct your eye line just above the camera lense or just to the side unless otherwise stated.

When doing the read, ask what frame, then you can do as much or as little as is required for the frame.
90% of film and TV castings its just you and the casting director, so don't expect expressive acting/reading abilities from the casting director, make sure you can act off anything they throw at you, if you have been asked for a specific accent, note that they will NOT being reading opposite you in an accent, so make sure that doesn't throw you off your required accent.

Read slower than you normally would, the most important person in that room is the person viewing you on the video. So make sure they can fully understand every word and can clearly see you. Ladies, if you have long hair, tuck it behind your ears.
Make sure nothing is obscuring your face, if you have a prop of sorts, make sure it is never obstructing your face, they want to see and hear you.. its not about what the casting director in the room thinks, its about the person watching back the tape. So make your performance for the tape.

I would avoid miming, ie opening the door, typing on a keyboard, making tea etc.. I would just ignore any stage directions and focus on the dialogue for the screen test. But this is just my opinion.

A big DO is to be prepared, know the script or sides and know a backstory for your character, the slightest thoughts your character has will show up on screen and those are the moments in your eyes which will make you stand out from the rest of the candidates. Its those subtle moments.
I would ALWAYS learn the lines off book, know them as well as you can.
Don't take anything else in the room with you except the excerpts from the script you have been given and any sort of prop you want (if you are on the phone for example).

Be prepared for ANYTHING, there could be lights, the director and producer could be reading opposite you rather than the casting director, in an ideal world you could have the other actor reading opposite you, you could be asked to read from a different section, to improvise, for every audition be prepared for anything and everything. But always know your frame.

And most importantly, be calm and enjoy it.