What is the test that should be preformed after assembling your own lanyard.
Maybe towing a car with it?
Sure, tow your car. I would do it in a short practice piece in which you have made an eye in each end. When it doesn't break or deform, you will have great confidence in your lanyard.
Here's a well-used lanyard splice. Views from opposite sides.
Note the geometry at the throat of the eye. Also note the strands are flattened and untwisted where they are tucked. Finally, this one was tapered by cutting away half of each strand where it emerged after the 5th tuck, then tucking the remaining halves one more time. This softens the transition between splice and rope.