Having a trial run of your makeup for your big day is extremely important. This is when you can fine tune the final look you want. It also allows for you to get to know your makeup artist, if you don't already know them. On top of this, you can test the staying power of the makeup and how your skin reacts to it.
Before the trial:
Google is your friend! Just type in 'bridal makeup' and go through the results. You will see things you love and things you hate. Take note of both. It's also helpful to create a collection of looks you do like. Below is a screenshot of a Pinterest board I made for a client.
I'd also suggest not trying any new products or treatments before your trial (and your wedding day). Whatever you do may cause a bad reaction with the makeup.
During the trial:
Speak up! If there's anything you like or don't like, say something. This will help guide your makeup artist. If you're not comfortable and happy with the way you look, the makeup will look horrible. That's the last thing any of us want. If there is something that is bothering you, best to discuss it with your makeup artist right then and there. That's exactly why you're having the trial.
After the trial:
Take note of how your skin feels. Are you itching anywhere? Does anything hurt? Do you like how the makeup looks hours after the application? Did the colour of your foundation change?
You can even test the limits of your makeup, if you want to. Go watch a sad movie and see if your makeup can withstand tears. Go exercise and see if it can survive sweat. Let your imagination go wild!