simple. there are two ways.
1) stars do not acclerate unless there is some force that is working on them. unless that force is a majorly large one and/or close to it, any acceleration that affects them is going to occur over millions of years before its measureable. therefore any change in your speed relative to them is your speed.
2) light has only one speed. that is why we know that the distance stars are a certain distance from us. it is also why we can know that they are moving towards us, or away from us. if light has different speeds then we cant know the distance of anything, from a star to an aircraft. measureing the difference between the speed of light coming towards you from in front and from in back and you will easily have the speed you are traveling.
a third way for slower speeds is to check the redshift/blueshift of stars in front of you and behind. since relatively speaking, the stars are not moving very fast, nor are they accelerating very fast, then any change is due to you, not them.
any first year high school algerbra student should be able to figure that out.
i will not claim to have a perfect understanding of the equations, but the ones that are easiest to understand are not that hard.
do you really not understand math?
