Lets say the speed of light is known to be 669,600,000 miles per hour.
We have a new space ship parked at a runway on Earth.
In our ship we have 20 bowling balls.
We take one bowling ball out of the ship and place it on the runway.
We mark this bowling ball "frame of reference one".
We get aboard our ship, our speed relative to bowling ball one is zero miles per hour.
Now we take off and fly into space, we accelerate to 75% of the speed of light
relative to bowling ball one.
We now set the auto pilot to maintain this speed, our ship maybe
experiencing drag.
Bowling ball two is placed out side the ship with it's own propulsion system, so it
can maintain it's current speed with the ship. We mark this bowling ball "frame of reference two".
Our speed relative to bowling ball two is zero miles per hour.
We accelerate to 75% of the speed of light relative to bowling ball two.
We now set the auto pilot to maintain this speed, our ship maybe experiencing drag.
Bowling ball three is placed out side the ship with it's own propulsion system, so it
can maintain it's current speed with the ship. We mark this bowling ball "frame of reference three".
Our speed relative to bowling ball three is zero miles per hour.
We accelerate to 75% of the speed of light relative to bowling ball three.
We repeat this process until we have used up all our bowling balls.
Don't worry because of relativity we are still traveling less then the speed
of light relative to bowling ball one, a square in our world is a rectangle back on earth
until we return to earth and park on the runway we started from.