Place four cones in a straight line on the ground in front of you, equidistant apart, as shown in the diagram.
When ready, run to the fourth cone then back to the first at 25% of your maximal pace.
Next, run to the fourth cone then back to the first at 50% of your maximal pace.
Next, run to the fourth cone then back to the first at 75% of your maximal pace.
Finally, sprint to the fourth cone then back to the first at 100% of your maximal pace.
Repeat.