This depends on the availability of root on your device:

This is the simplest way, provided that you have done rooting. Just install GLTools APK and grant root to it when it launches. Then you will be all set.

Here is where things get more complicated. Usually apps are not allowed to interact with others without root on Android, so you will have to put GLTools into some more permissive environment. There are two common environments that can be used: Virtual Machines and Virtual Spaces.

This is the most suitable way to use GLTools without (real) root. Some of these VMs provide root, so installing GLTools is as simple as in a regular rooted case, except for one detail: GLTools and the games it will affect should be installed inside VM.
You can download VMs by the following links: F1 VM, VPhoneGaGa, VMOS and Virtual Android.

Using virtual spaces is the second way to use GLTools without root. It requires you to install some proper virtual space first: it has to have debuggable flag set. Such virtual spaces are usually used for GameGuardian app, so you can find patched virtual spaces on its forum. One of the suitable virtual spaces is VirtualXposed.
Note that GLTools and the games it will affect should also be installed inside the same virtual space.

First of all, configure some game with settings you would like to enable. To do that, select "Configure games" tab and click on one you would like to configure. Then change some settings you like and click "back" to save them and return.
Now it's time to look what happens. Use game menu - launch game or return to the "Home" tab and launch game from there by clicking its icon.
GLTools will only be active if game is launched from its menus, settings won't work if you launch the game directly from Android launcher.

First of all, Premium removes all ads entirely, but it also provides some useful features. You may find a requirement to launch GLTools each time you run a game a bit tiresome. Premium provides a solution for this problem: it lets you create shortcuts that will automatically launch your game with GLTools enabled making this almost the same to the regular game launch.
To create shortcuts, visit "Create game shortcuts" side menu or click "Add shortcut" in the game menu.

The main way to do that at the moment is getting Premium in our companion app, Premium for main GLTools app will be provided as a gift. It will be detected automatically after purchase, but if it does not, click "Clain your gift key" button inside companion and copy license key from the dialog. Then license key can be entered by clicking left menu - Get Premium - Enter Key. GLTools will require Internet connection to check your license key periodically.
We advise you to save your license key in a reliable place. Note that in case of misuse (e.g. sharing) this license key may be revoked, so make sure to keep the key private.

This question doesn't have an easy answer, the best way to know is trying. We have divided the settings into the following categories:

• Quality - settings, that affect how good the picture looks. Increasing quality usually comes at a performance cost
• Performance - settings, that affect rendering speed. Performance gains are usually not free in terms of quality, but some (like GLSL optimizer) may come with no disadvantages
• Memory - settings, that control memory usage of the game. If you have enough RAM, these settings are useless, but if you are low on RAM, they may come handy
• Compatibility - settings, that improve your games compatibility. Use these settings if the game doesn't run for some reason (for example, it is designed for much older Android versions). Some of these settings are marked "Dangerous": enabling them may break a previously-working game
• Battery - settings, that control your CPU and GPU utilization. Use them if you would like to sacrifice FPS and quality for battery life

There are also extremely useful settings that don't belong to any category on their own: Replace GPU, CPU, RAM and device info. These settings can be used to disguise your device as another one: if the game treats some devices better than other, this may result in quality, performance and compatibility gains. Don't forget to enable "Replace GPU info" and "Replace device, CPU and RAM info" for respective disguise. Usually "Replace GPU info" alone is enough.