Desktop

Bluetooth settings are in the upper-right corner, click it and select Device.

Click Search to scan for Bluetooth devices nearby.

Select the device that you want to pair with.

Server

Enable Bluetooth:

sudo rfkill unblock bluetooth
sudo hciconfig hci0 up

Launch bluetoothctl:

$ sudo bluetoothctl
[NEW] Controller 43:54:A2:00:1F:AC Khadas [default]
Agent registered
[bluetooth]#

Setup:

[bluetooth]# agent on
[bluetooth]# default-agent
[bluetooth]# power on
[bluetooth]# discoverable on
[bluetooth]# pairable on
[bluetooth]# scan on

Scan for Bluetooth devices:

[bluetooth]# scan on
Discovery started
[CHG] Controller 43:54:A2:00:1F:AC Discovering: yes
[NEW] Device 46:04:25:5F:1E:8D 46-04-25-5F-1E-8D
[NEW] Device 8C:EB:C6:E7:2E:33 Khadas

Show connected Bluetooth devices:

[bluetooth]# devices
Device 46:04:25:5F:1E:8D 46-04-25-5F-1E-8D
Device 8C:EB:C6:E7:2E:33 Khadas
Device 9C:FB:D5:0D:91:47 9C-FB-D5-0D-91-47
[NEW] Device AC:83:F3:DD:D4:E1 AC-83-F3-DD-D4-E1
[CHG] Device AC:83:F3:DD:D4:E1 Name: LibreELEC
[CHG] Device AC:83:F3:DD:D4:E1 Alias: LibreELEC

Pair with a device:

[bluetooth]# connect <device_addr>

Exit bluetoothctl:

[bluetooth]# quit
Agent unregistered
[DEL] Controller 43:54:A2:00:1F:AC Khadas [default]
khadas@Khadas:~$