Some things to say first:

• At moment of testing the commands and writing this, I am using a ubuntu 18.04.3.
• I don't have a Ethernet cable ¯\_㋡_/¯
• My raspberry is the 3 Model B+ and my sdcard is a SAMSUNG EVO Select microSDXC 128 GB
• This is my first post (treating it more like a "shared note") on dev.to community 🎉

1. Download the ubuntu image 18.04.3-preinstalled-server-arm64+raspi3.img.xz (link taken found here)
2. Flash the sdcard using balenaEtcher

3. sudo nano /media/wesley/writable/etc/netplan/50-cloud-init.yaml
   It should contain:
   

   network:
       version: 2
       ethernets:
           eth0:
               optional: true
               dhcp4: true
       wifis:
           wlan0:
               optional: true
               access-points:
                   "YOUR-SSID-NAME":
                       password: "YOUR-NETWORK-PASSWORD"
               dhcp4: true
   

   Make sure to have navigated to writable partition, otherwise nano will complain the folder does not exist.

4. sudo nano /media/wesley/writable/etc/cloud/cloud.cfg

   Should be like this:
   

   # The top level settings are used as module
   # and system configuration.
   
   # A set of users which may be applied and/or used by various modules
   # when a 'default' entry is found it will reference the 'default_user'
   # from the distro configuration specified below
   users:
      - default
   # If this is set, 'root' will not be able to ssh in and they
   # will get a message to login instead as the default $user
   disable_root: false
   
   # This will cause the set+update hostname module to not operate (if true)
   preserve_hostname: false
   hostname: YOUR-RASPBERRYPI-NAME
   
   # Example datasource config
   # datasource:
   #    Ec2:
   #      metadata_urls: [ 'blah.com' ]
   #      timeout: 5 # (defaults to 50 seconds)
   #      max_wait: 10 # (defaults to 120 seconds)
   
   # The modules that run in the 'init' stage
   cloud_init_modules:
    - migrator
    - seed_random
    - bootcmd
    - write-files
    - growpart
    - resizefs
    - disk_setup
    - mounts
    - set_hostname
    - update_hostname
    - update_etc_hosts
    - ca-certs
    - rsyslog
    - users-groups
    - ssh
   
   # The modules that run in the 'config' stage
   cloud_config_modules:
   # Emit the cloud config ready event
   # this can be used by upstart jobs for 'start on cloud-config'.
    - emit_upstart
    - snap
    - snap_config  # DEPRECATED- Drop in version 18.2
    - ssh-import-id
    - locale
    - set-passwords
    - grub-dpkg
    - apt-pipelining
    - apt-configure
    - ubuntu-advantage
    - ntp
    - timezone
    - disable-ec2-metadata
    - runcmd
    - byobu
   
   # The modules that run in the 'final' stage
   cloud_final_modules:
    - snappy  # DEPRECATED- Drop in version 18.2
    - package-update-upgrade-install
    - fan
    - landscape
    - lxd
    - ubuntu-drivers
    - puppet
    - chef
    - mcollective
    - salt-minion
    - rightscale_userdata
    - scripts-vendor
    - scripts-per-once
    - scripts-per-boot
    - scripts-per-instance
    - scripts-user
    - ssh-authkey-fingerprints
    - keys-to-console
    - phone-home
    - final-message
    - power-state-change
   
   # System and/or distro specific settings
   # (not accessible to handlers/transforms)
   system_info:
      # This will affect which distro class gets used
      distro: ubuntu
      # Default user name + that default users groups (if added/used)
      default_user:
        name: pi
        lock_passwd: True
        gecos: Pi
        groups: [adm, audio, cdrom, dialout, dip, floppy, lxd, netdev, plugdev, sudo, video]
        sudo: ["ALL=(ALL) NOPASSWD:ALL"]
        shell: /bin/bash
        ssh_authorized_keys:
          - "CONTENT-OF-YOUR-SSH-PUB-KEY-HERE"
      # Automatically discover the best ntp_client
      ntp_client: auto
      # Other config here will be given to the distro class and/or path classes
      paths:
         cloud_dir: /var/lib/cloud/
         templates_dir: /etc/cloud/templates/
         upstart_dir: /etc/init/
      package_mirrors:
        - arches: [i386, amd64]
          failsafe:
            primary: http://archive.ubuntu.com/ubuntu
            security: http://security.ubuntu.com/ubuntu
          search:
            primary:
              - http://%(ec2_region)s.ec2.archive.ubuntu.com/ubuntu/
              - http://%(availability_zone)s.clouds.archive.ubuntu.com/ubuntu/
              - http://%(region)s.clouds.archive.ubuntu.com/ubuntu/
            security: []
        - arches: [arm64, armel, armhf]
          failsafe:
            primary: http://ports.ubuntu.com/ubuntu-ports
            security: http://ports.ubuntu.com/ubuntu-ports
          search:
            primary:
              - http://%(ec2_region)s.ec2.ports.ubuntu.com/ubuntu-ports/
              - http://%(availability_zone)s.clouds.ports.ubuntu.com/ubuntu-ports/
              - http://%(region)s.clouds.ports.ubuntu.com/ubuntu-ports/
            security: []
        - arches: [default]
          failsafe:
            primary: http://ports.ubuntu.com/ubuntu-ports
            security: http://ports.ubuntu.com/ubuntu-ports
      ssh_svcname: ssh
   

5. Eject from your computer, inject the sdcard on the raspberry, connect the power and wait for about one minute.

6. ssh pi@YOUR-RASPBERRYPI-NAME

ℹ️ first time you login it will ask for a password (type ubuntu) and then to change to a new password.

Many thanks to l --marc l user on stackoverflow. Steps on this post are 90% from his answer here. The other 10% comes from the cloud-init docs