Please download Etcher and install for your Apple or Linux machine from here:
Then download the image file for pi-top os from here:
Then you will be able to see the zip file in your downloads folder.
Insert the SD card you would like to use, Etcher will automatically look for the card, select it then click flash.
The process will take some time dependant on your machine.
When it has finished, remove from your machine and you will be able to use your freshly installed copy of pi-top OS in your CEED or pi-top.