Nice on the Njoy plug, I have the Pure Plug 2.0, it's a good investment
For your dare: Start your day with the biggest plug you can manage to wear continuously. The Njoy if you must, but if you can, go for the large silicone one -- go big or go home! Warm up with a smaller toy if needed.
Go about your day as you would normally (work, school, whatever). Choose a day where you have to leave the house. At one point during your day, go out and buy an insertable object that is slightly larger than your silicone plug. This can be an actual toy, but better if you misappropriate something. A vegetable? A tool? Be creative. You know where this is going, choose something large, but with a reasonable expectation of your butt being able to take it after some instruction.
When you get home, take out your plug and finger yourself with two fingers, one on each hand. How open does it feel? Can you "pull apart" your butthole? Start masturbating and edging at this point, but don't cum!
Lube your plug up again and reinsert it, then take it out fully. Do this 10 times at least. Repeat the fingering. More open? Yes? Reward yourself by starting to wear the ball gag. Continue masturbating, no orgasm.
Now take the object you've bought and lube it up (use a condom if you want). Push it up your butt. It's OK if this takes you a while, don't give up. If you can't manage, use your silicone plug to stretch more. Bite down on that ball gag to give it your all. You're only allowed to cum after inserting the object fully (as far as it will reasonably go) and edging 10 times with it in your butt.
If you cannot manage that, you must wear the silicone plug overnight and may cum in the morning before taking it out.