Take a look at the bottom of the fitting and you will see the O-ring. It is pre-installed on the fittings.
Ideally, the fittings should be screwed in without the compression nut attached. This also makes it easier to check whether they are properly installed. After that, insert the tube and tighten the compression nut.
Unfortunately, there is no real expert trick for this. However, it should not normally be that difficult.
Of course, you can also install the Y-splitter on the IN port. You just need to keep in mind that the drain valve should ideally be positioned at the lowest point of the system. Otherwise, draining the loop will be quite difficult.
Ideally, the fittings should be screwed in without the compression nut attached. This also makes it easier to check whether they are properly installed. After that, insert the tube and tighten the compression nut.
Unfortunately, there is no real expert trick for this. However, it should not normally be that difficult.
Of course, you can also install the Y-splitter on the IN port. You just need to keep in mind that the drain valve should ideally be positioned at the lowest point of the system. Otherwise, draining the loop will be quite difficult.