Going off the most accepted answer, but whose author unfortunately said modifying the PATH is "hard" - it's not at all. It's very easy. I just did it, and you can too!
Here's what to do:
Navigate to your sublime text directory in Windows Explorer and copy the full path from the path bar.
For example, I have a few development tools set up as portable applications in Dropbox, so I copied the path:
C:\Users\username\Dropbox\Programs\Sublime Text Build 3114 x64
Go to Control panel-> System-> Advanced System Settings-> Environment Variables
In the resulting window, navigate to "path" in the bottom window, highlight it and click "Edit".
In the window that appears, click "New", paste in your Sublime Text path, and click "OK".
Now open CMD or Powershell and type subl
Sublime Text window should open. Enjoy!
Note: I noticed someone else answered this in a similar fashion, but used a fixed location for Sublime Text. That may work for some people, but for others the directions will be erroneous. The fact is, the Sublime Text location can be anywhere on the drive, and knowing where it is and copying that particular location is one of the most important parts of assigning the PATH variable.