Best bet would be to work up from a multi level dildo such as a "tomatoes on stick" which would be from around 3.5cm diametre to the 5th biggest "tomato" being around 5cm. This will set you up for a bit of stretching and the toys are incredibly cheap. From there, grab a 5cm knot dildo and if you practiced with tomatoes on stick, then you will be more comfortable taking the 5cm knot and the pop will be great. If you wanna practice further, a horse dildo will help you become used to the feeling of the knot, especially as it pops in and you get accustomed to the feeling of something very large inside you because the knot is at the end, it is much easier to control.
Just don't put too much weight on because if it pops when you're not expecting it, then you WILL know it's in! The first time it happened to me, I was in a confused state of euphoria lol. Then move onto the dog dildo and you should be able to handle the knot without it feeling easy. Best advice is take it slow and relax, don't rush and remember , lube is your friend!