First of all, you need to vote to get access to world edit (unless you're timeless) Secondly, you need to mark the area you want to download. Make sure you include the whole build. To do so you just right and left click on two blocks surrounding the build using a wooden axe. (can also be done using the commands //pos1 and //pos2) After that, you have to copy your build to your clipboard using //copy. Lastly, all you have to do is download the build using //download, clicking on the link and after that, it should start downloading. (click on "Click here if your download doesn't start automatically" in case it doesn't start downloading)