do you ever play with the ambient energy around you?
do you ever mould and shape it, or use it in a working?
it's done the exact same way, literally just reach out to the energy you want, and pull it into yourself
for your own safety though, you will want to filter it, if you are going to absorb it
there are two main things you can do with the energy you pull from somewhere/one
you can consume it, make it your own energy - this has risks, it can be very addictive, and if you do not filter it before hand, you will need to cleanse and balance afterwards, to clean out the gunk that will come with
or you can ground it out, return it to the earth
edit to add - I really cannot explain it any better, for myself at least, it is as much an instinctive, automatic process as chewing is.