Okay, so if the gray hose is loose already, pulling it out of the piping and pushing it to the side for a bit might make things a little easier.

After that, you should be able to unscrew the white collar just like a bottle cap. If it's too big for your grip, a strap wrench might help.

Once the collar is off you'll want to push the gray hose through it like a fat finger through a ring. The gray plastic should be flexible enough to withstand a little deformation; you may need to bend it a little bit to get it to fit through the collar.

Once the collar is on the hose it should be "captured" by that little lip on the hose, by which I mean it should slide around but it shouldn't slide right off without a little effort. Screwing the collar on to the piping with the hose through it like that should squeeze the lip of the hose to hold it in place.

When tightening the union nut, only tighten to the point that it's snug. Not because you might break it, but because anything more than that is probably unnecessary. After that you should keep an eye on it at least during the next drain cycle and check to make sure nothing is vibrating or pulling loose.

Does that help?