You can perform this lunge from the floor, off a box (5-10 inches), with or without load or any combination of these things.
Take a big step back.
Allow your torso to hinge forward slightly as you sink down.
Do not allow your front knee to move forward beyond your toes.
Before you attempt this lunge, make sure you understand and can perform the basic skills associated with the Alternating Reverse Lunge (the ability to sink your body down into the lunge, maintain a steady front knee and press strongly down into your pillar leg).
Do not allow your front knee to wiggle into the midline. If you cannot keep your front knee stable, you might need to continue working on the Split Squat exercise and master that moment before coming back to this lunge.
Choose a load and/or box height that allows you to complete 5 - 10 repetitions. These repetitions should challenge you while still allowing to maintain solid, good technique (although not necessarily perfect technique) and work within your "smiling pace."
The pictures below show just some of the many ways you can add load to this exercise.
As is the case with most movements, you can do this movement outside, in an airport, at your office, or wherever you happen to find yourself. Your movement doesn't always need to be limited to a gym setting or a specific workout hour. Get creative, have fun and keep moving!