Some general instructions are provided here for flashing Plasma Mobile onto your device.
Currently, for testing, you may also wish to install a newer version of Plasma Mobile. ./pm-flash -p edge will install an "edge" rootfs, which has been rebased on Neon 18.04 and has much newer packages. These are rebuilt regularly. Before reporting bugs, we recommend you test with this to see if the issues still occur with the latest code.
After flashing, the script now automatically resizes your root partition for you, and you can connect to Wi-Fi from inside Plasma Mobile easily. Extra steps are no longer necessary. If your Plasma Mobile device has a USB connection to your PC, you can connect to it via SSH with ssh firstname.lastname@example.org
By extension, SFTP is also supported, so if you want to copy a file from your PC to the device, you can either add it as a remote folder inside Dolphin, or use something like scp /path/to/file email@example.com:/home/phablet