With years of experience in various FPS games this the routine i follow
1. Warm-up my body; any exercise would help. I do Push ups.
2. Jump onto Kovak and train my reflexes. Any other aim trainer can work too.
3. Maintain consistent settings across all games. Many sensitivity converters can help with this. Also, optimize your settings in accordance with your needs, such as the right aim dot sight, colors, any night light, or other brightness/contrast corrections you need to avoid strain on your years. You should be comfortable.
4. While open ques or quick plays are good for casual play, you can only improve in ranked modes, You have to challenge yourself, even though it can lead to agression at times.
5. Keep yourself relaxed and don't be too hard on yourself; more nervousness and anger can lead to more mistakes.
6. A personal tip is keep your monitor close and make sure your hand has enough space for movement.
7. Lastly keep practicing and one day you'll have a elite level reaction time.
Good Luck