Poly pipe joiners are easy to use and learning how to install leak-free poly pipe joiners is also easy too:
When pushing fittings onto the pipe, ensure the pipe is inserted fully. If the fitting is only partially inserted onto the pipe it will not create a watertight seal. When the pipe is traveling through the fitting there will be some resistance. This means you can think the pipe has been pushed all the way into the fitting. Making a witness mark will help show how far the pipe needs to be inserted in order to reach half way into the fitting.
In order to seal tightly inside the fitting, pipe ends need a clean, straight cut. If the end is jagged or uneven, it will not create a watertight seal
If the tubing is cut on an angle, it is unlikely to seal and could also nick the O-ring
One way to avoid these issues is to cut pipe with a pipe cutter rather than a blade or saw