Here, we don't mean channeling in the sense of channeling spirits or other non-corporeal entities but rather the idea of taking something (such as a negative emotion or psychological state) and transforming it into something else entirely.
A good example here would be of taking anger and using it to exercise. Exercise releases a flood of feel-good chemicals in our body, and it's a great way to channel the energy that's generated from the adrenaline of anger.
Sadness is a little trickier: for sadness, sometimes we just have to ride things out and try to rest as much as we can.
But then again, many people find that art is the way they express their emotions, up to and including sadness. I've heard of one woman who had a particularly sad event in her life, and she channeled that emotion in producing massive amounts of poetry.
There are various scenarios one might come up with. In fact, I'm giving you, the reader, a bit of homework here: come up with some situation in your life where you had a negative state of mind, and you channeled that into something else. I'd love to hear it, and so would everyone else!
The transformation of an emotional state may take more than one avenue to change. Anger may not resolved simply by expressing it; other underlying issues may have to be logically sorted out. I've stated elsewhere that I'm a huge fan of thought-restructuring; it's a way of adjusting our outlook and our attitude that isn't a flimsy attempt to simply deny how we feel.
Telling yourself that you feel some way that you don't has never worked for me; it's like trying to lie to myself, and I see through it. But I can try to imagine how I would feel is things were different, and then try to see things from a different point of view.
Anyway. I hope things are going well for everyone, and I hope you find some avenue of transformation for your more difficult experiences in life.