If your pipe is going to attach from a gutter, down to a water butt, like ours, you first need to make sure you have an access hole in the lid of your water butt.
Now, push the top of the pipe up over the end of the gutter piece, and insert the bottom end of the pipe into the water butt access hole.
If you find it is too loose and may flop out, you may need to add another bottle.
If on the other hand it is too tight to get into place, you may simply need to push the bottles a little closer together.
Attaching this way requires no additional fixings, it is simply wedged between the gutter and the lid and because there is very little give, it holds fast, but you may choose to secure it further if you wish.
This coincides with the other major flaw (or pro, depending on your needs) of the BeFree: its size. Some people — trail runners with access to flowing water — will love a tiny bottle that can be refilled and filtered at a moment’s notice and then stashed quickly in a vest or small pack. But most people carry more than .6 liters of water on them during a day hike. Hydrapak sells the Seeker series of soft flasks, which do work with the BeFree threads, and come come in 2L and 3L sizes. This increases your capacity, but requires you to buy another bottle, which is not ideal. You can purchase the filter/cap on its own for $25 and a Seeker flask that meets your needs separately, but there should really be a BeFree filter in multiple sizes to get the most versatility.