The first place to start would be to check the roof bar/fitting kit instructions as you will often find this information here.
Plain roof: If your bars attach around the door frame you'll need to check the instructions, they will tell you exactly where to fit the bars.
Fixing points: This should be fairly obvious! If your vehicle has more than two sets of fixing points you can check the instructions to see which ones the bars are designed to fit. If you have a choice, you'll need to decide which ones are best for the gear you are carrying (e.g. fitting a roof box further forwards so it doesn't conflict with the tailgate).
Roof rails/flush rails/T-track slots: Again, check the instructions in case there are specific places on your rails to fit the bar. If not, we recommend positioning the bars around 70cm apart for gear such as roof boxes and bike carriers. You may want to increase this distance if you are carrying longer gear.
If you bought the bars from us and have lost the instructions, just contact us. We can supply you with a new set.