First you will need to go online and login to your office 365 account. From there will need to click install office 2016. This will need to be done on a windows computer as the website checks to see what operating system you are on and only allows you to click it if you are on a mac or windows computer (don't use mac or else you'll have the mac version which won't work with crossover). When you click the link it will download an executable file onto your windows computer. You will then need to transfer this onto your Chromebook either via USB or via a cloud service (I uploaded it to my google drive).
Once you have successfully transferred the executable file you will need to move it into your Chromebook's downloads folder. Now you need to open the crossover app and search for office 2016. Click install and it'll ask you to provide an installer. If it says it can't find an installer there should be an option to search your downloads. Either way it should show you your downloads folder, which should include the executable file. Then you will need to click the install button next to the file.
During the installation a big orange box will appear on the screen saying it is installing office 2016. Once that has done it will close. When I did it, crossover then remained saying it was installing software dependencies for over an hour. I decided to cancel it and there seems to be no impact of doing this. All the applications seem to work fine, despite me cancelling the installation of the software dependencies. It is possible that crossover was simply attempting to download non-existent dependencies or just got stuck. If you have this problem and you don't feel comfortable cancelling the operation I advise you simply contact code-weavers. However it may be a better option to simply cancel it and see if the applications work.
I hope this helps,