When you connect the Kobo to your PC (I'll assume Windows here), Windows assigns at least one drive letter to the storage on it. If you happen to have an SD card installed in the slot of your Kobo, a second drive letter is assigned to it.
I'd suggest that you create a folder in the root of the drive (say eBooks), and then just copy all the books you want to sideload into that folder.
The device can either be ejected via Kobo Desktop, or via the Removable Device icon that Windows displays in the system tray. (See
https://www.winhelp.us/use-removable...n-windows.html for what this looks like in different Windows versions).
Once you eject the device and disconnect the cable, the Kobo will display a progress bar as it processes the new content.