How to make Fennel juice?
You can buy fennel seed extract, but I can't find instructions to make it. I assume you could just crush the seeds, put them in vodka and strain in a week or two.

If you don't want to use alcohol, you could also use propylene glycol or glycerine.

Another thing you might consider is using vinegar the same way. That could certainly be an interesting accent to certain drinks.

Any kind of attempt to juice the seeds isn't going to work. Extraction should work, I'll experiment with it. I'll update this post in a couple of weeks if no one beats me to it.

EDIT: It says on WIKI that you can make a tea with fennel seeds. By golly it works! I used slightly crushed fennel seeds and added boiling water. Once it cooled to drinking temperature, I strained it. I'm drinking it now, it tastes like fennel seeds. It's quite nice with a touch of honey.

Later EDIT: I just checked on the fennel seed extract in vodka and the vinegar. Both seem to be working just fine. I think the vinegar has some real potential. Maybe not so much with fruit, but perhaps with vegetable juices and/or kefir. I'm also thinking that it is going to make a great salad dressing.

UPDATE: The extract made with vodka is exactly how I expected it to be. It's a bit boozy (of course) but it tastes like fennel seeds. A few drops of this could easily replace actual fennel seeds in any recipe I can think of.

The vinegar is another story entirely. This stuff is awesome. I'm not yet sure exactly how I am going to use it, but I will, and I'll make it again. Furthermore, this has inspired me to make an infused vinegar using all of the whole spices that I still have from my most recent adventure with pho (Vietamese noodle soup). I can imagine it as a secret ingredient in all kinds of goodness. Thanks for the question which in turn inspired good stuff!