Well there are several things you can try.
First give yourself at least 2 enemas or even three until you get a no solids clear return. When you get a clear return, then you know your colon is clear or mostly clear. You don't want your toy to have to push through poop.
After achieving a clear return, here is a tip that I have recently tried that does seem to help getting a toy moving through the twisty chicane of the sigmoid. Give yourself a coconut oil enema- just coconut oil about a cup full and retain it. You might be surprised that it won't be that hard to retain if it's just a cup full. Also, while taking the coconut oil enema, elevate your hips so the oil drains into your sigmoid and descending colon. Now you have lubricated your colon.
You are ready to insert your toy- I would also recommend using coconut oil as lube. Slowly work the toy in until you feel it hits a stopping point. Work the toy slowly back and forth and see if you can make progress and get past your stopping point. DO NOT KEEP PUSHING IF YOU FEEL PAIN- STOP AND PULL BACK. Be patient and realize you may not get past your sigmoid, but keep doing slow advances and pull backs. If you can advance without pain, continue to go slow. This process can take 30 minutes or more.
When I do deep anal penetration with one of my colon tubes actually more of colon snake, I can see the bulge under my skin just off to the side of my left hip bone- that's when I know I am in the sigmoid and I can actually direct the tube with my fingers as it pushes through. Once through it moves easily up the descending colon.
I should also say the toy/colon snake etc. is also key. The softness/flexibility plays a big part in its ability to navigate through your sigmoid. Too stiff and it hits the wall and won't advance. Too soft and it can bend and double back and go back into the rectum. Either situation is very difficult to tell what's happening.
Deep anal penetration isn't easy so you have to be able to accept defeat on some occasions and live to try again another day.