Hi ! It's quite easy if you use a couple of calibre plugins to remove the drm first. You could check apprenticealf blog (
https://apprenticealf.wordpress.com) for a long explanation. But to make a long story short, for the kobo, you have to follow this pattern :
1. got a adobe ID and install adobe digital edition on your mac
2. authorize the kobo to use adobe DRM (run adobe digital edition, plug the kobo, connect to the mac, right clic on the device inside ADE and "authorize")
3. go to your online kobo account, and download the free epub or the adobe drm epub (you'll get an ascm file)
4. open and download it with ADE, select it and show the file in the Finder
5. import the file in calibre where you have previously install the apprenticealf DeDrm plugins
6. you'll get a drm-free epub which continue to be considered by ADE as the one you bought (because, in fact, this is the case).
If you need a step by step help, just ask ; i bought 2 books yesterday that i haven't dedrm yet. I could do the thing step by step with screens.