No video here. But my simple way of getting this achievement is as follows.
LVL 14 in Portal.
Make your way from the starting chamber down the hallway.
Turn left and go up the fancy moving stairs.
At the top, stop,turn around, look back and see that the stairs drop down.
On one side of you (to your right) is a platform with the companion cube, ignore it.
On the opposite side of you from the companion cube (to your left) is a smooth wall. One end joins the back wall, in the rear corner, and at the other end it joins a narrow - 1 block wide wall, in the front corner- next to the pit where the stairs once were.
Basically put a portal onto the back wall in the rear corner way up at the top so that it's facing that small 1 block wide front wall.
Face the pit where the stairs were, step off the edge and put the 2nd portal into the floor where you are falling. You will come out of your first portal at a decent speed and hit that 1 block wide front wall, before dropping to the floor in the front corner.
If you are quick you can re-shoot your 2nd portal to be on the floor in that front corner. A loop results and in no time you have the 300 ft jump. (no time being really about 30-60secs)