This is very, VERY important, especially if you are considering doing Astral Travel or any sort of out of body travelling. This, well, to be put simply, grounds you to the earth.
What you can do to ground yourself is rather simple. This is different for every person so I will list a few things that you could do.
METHOD 1: Imagine you are sitting in a forest/park/by a river/etc, somewhere nature related and relax. If you're not relaxed, this may end up being uncomfortable or it may not work in general. Now, imagine that you're a tree sprout and you're rooting yourself in the earth. Imagine yourself growing tall and deeply rooted into the earth. After that, imagine that you turn into the roots and the tree slowly disappears. AFter that, you should be done. IF you do not feel secure or like youre actually rooted then you may need to try again.
METHOD 2: relax and concentrate. Now, this is slightly similar to Method 1, but I find it helpful. You're sitting wherever you please and your body is starting to become one with the earth. You are in the earth and one with it, imagine yourself with the roots and with everything around. Once you feel secure or firmly grounded, you're good.
There are many other things that you could do for grounding, but these are the two methods that I generally use and have seen in the community. If you do have any ideas or methods that you have used, please feel free to message me ^_^ I am always open to new ideas!