If you have unergonomic furniture but you can’t change it (e.g. getting a taller chair, or putting some kind of booster seat on top of your existing chair (a company called HumanTool makes one I like), switching to a lower or adjustable-height desk, or installing an under-desk keyboard tray) or switch to a different keyboard, then the most important thing is to position/orient the keyboard so that your wrists are not bent while you are typing. Having bent wrists (either extended or flexed) is the most effective way to give yourself RSI.

To keep your wrists straight, pull the keyboard fairly close to the edge of the desk, then tilt the keyboard so that the top surface is parallel to your forearms, when you have your arms in a relaxed position with your upper arms close to your torso and your shoulders back.

For a high desk/low chair, you need to tilt the keyboard up at the back end. The taller the desk relative to your torso, the more aggressive the tilt needs to be.