The below assumes you have set up the tank and filled it with water which has been dechlorinated (it that is needed).
1. Shake the bottle of SafeStart. (I would have told you to buy Dr. Tim's One and Only as a first choice, you found the next best option.)
2. Turn off tank lights for the next 24 hours..
3. Leave filter running
4. Pour the entire bottle into the tank.
5. Add ammonia to produce 2 ppm of ammonia.
6. Test in 24 hours for ammonia. If it is 0, you can add a full stocking of fish after a water change of at least 50%. If not,
7. Keep testing until ammonia hits 0 and repeat steps 5-7 until you can ytest 0 ammonia in 24 hours.
You can use household ammonia with no additives or you can use ammonium chloride. The easiest way to do this is to buy a bottle of Dr. Tim's Ammonium chloride.
https://store.drtimsaquatics.com/Ammonium-Chloride-Solution-for-Fishless-Cycling.html you can get it on Amazon as well.
Here is what Tetra instructs you to do:
Do not waste your time and money following their maintenance stuff, it is only making them money because you should not need it.
Do not add fish until you know it is safe. If the bottled bacteria has not been properly kept by the seller it can die. If it has been in the bottle too long it can degrade (think about 6 months at room temp and about 1 year if kept refrigerated). In such cases it does nothing good. No knowing it has worked means you cannot be sure the product was viable when you received it. Adding the ammonia as I suggested above will tell you if the tank is safe or not.
In a healthy established tank the nitrifying bacteria can be found all over, not just in a filter. It will be in the top 1/2 inch of unplanted substrates and much deeper in planted ones. It can be on any hard surface in a tank which is out of the light. The bacteria do not like the light. You pour the water into the tank to seed the bacteria everywhere. It will establish best where it gets what it needs- water with ammonia/nitrite/oxygen/iron etc. the bacteria are basically immobile and all things they need must come to them, they do not actively more about seeking things. This is why you pour it into the tank and not just the filter.