1- First, rasp the pipes tip as picture

2- Reassemble the fitting components and put the nut and spilt ring respectively.

3- Push and twist the appropriate pipe in to the desired fitting's body in a way that it passes through the bushing, so the tip of the pipe faces the inner part of the body. Lubricating of pipe and O-ring will help you. Push the split ring inward till it touches the bushing, widening of the split ring can do the movement easier.

4- Wind the nut by hand and tighten it with proper wrench.