First, I'm going to define a few terms for you that you'll need to know.
- TAS - Tool Assisted Speedrun, which means that the speedrun was made using advanced tools and probably contains actions that are difficult/impossible for humans to perform
- RTA - Real Time Attack, which means that a human player sat down with a controller and played the game
- RTA Viable - means that the speedrunning community considers a trick or route to be possible to be performed in a RTA run
- BitDW - Bowser in the Dark World, the first Bowser World, behind the 8 Star Door
- BitFS - Bowser in the Fire Sea, the second Bowser World, behind the 30 Star Door
- DDD - Dire Dire Docks, the level behind the 30 Star Door that you must collect the first star in to access BitFS
- BLJ - Backwards Long Jump, the name of the trick that you're describing
- Mips the Rabbit - this is the rabbit that you have to catch in the Basement of the Castle Twice in normal playthroughs
- 16 Star - a category in Super Mario 64 speedrunning where the player collects 15 stars, uses a glitch using Mips the Rabbit to glitch through the 30 Star door, beat DDD, BitFS, and then uses BLJs to skip the 50 Star Door and the Infinite Staircase
- 0 Star - a category in Super Mario 64 speedrunning where the player uses BLJs to skip the 8 Star Door, the 30 Star Door, DDD, the 50 Star Door, and the Infinite Staircase.
I used to run "16 Star" in Super Mario 64 years ago. Back then, the 8 Star Door BLJ was not considered skippable outside of TAS and the 30 Star Door required using a different glitch to skip using Mips the Rabbit.
I've long since quit speed running (and I wasn't very good at it even when I was doing it) but since then the "0 Star" route has become RTA viable. You can find the current RTA world record here.
I bring this all up because I'm guessing you're trying to do either the first or second BLJ in "0 Star". These are widely considered to be the 2 hardest BLJs in the route. Furthermore, the first BLJ you see in the video you linked isn't the one that is used in the current RTA world record. That video is a TAS run and the BLJ they use is much more difficult than the one in the link above.
The easiest BLJ to perform, by far, is the Infinite Staircase BLJ, which just so happens to be the last BLJ in the run. The second easiest is the 50 Star Door BLJ, which is the second to last BLJ. I would recommend starting a new game, getting to 50 Stars, and then practicing on the Infinite Staircase before even attempting to try the 8 or 30 star door skips. These two BLJs require no more skill than button mashing, and the Infinite Staircase gives you unlimited space to trigger the BLJ.
If you're still insistent on trying the first two BLJs...
Also note that if you are playing using the Shindou, IQue, or Japanese Virtual Console versions of the game, the glitch has been patched out of the game and is not possible in these versions.