I often read posts or see videos where decorators explain that they use melted chocolate to "glue" either decorations or certain parts of the cake together. How? Do you just pour it on? Pipe it? Use a brush?

Depends on how much I need. I have used a toothpick and a brush before. It dries faster than royal icing which is another good glue option. It could also be piped on...just depends on the amount needed.

I use it all the time for attaching decorations especially gumpaste that are already hardened and set. I melt a few candy melts (whatever color works best with your design) in a small bowl in the microwave. Then, I dab a bit on the back with the tip of a spatula and attach to the cake. This works with fondant on fondant/gumpaste. I wouldn't think it works with buttercream. I always use water to attach to buttercream.

I pipe it, but a brush would work also.
Pouring it on might get a little messy.

For the large bows on my ivory cake with brown bows in my pics I used a piping bag to attach the gumpaste bows to each other with melted chocolate. Held together really good.

I've been using piping gel or icing to attach decorations, but I thought I'd try melted chocolate for things that I really need to hold well onto my fondant.

Gabby...since piping gel never "sets", I think you will like how the chocolate works! Good luck.

Another thing...I ususally pop the decoration in the freezer if it's something like a # or letter topper that I've attached toothpicks to it with the candy melts. Dries so fast!