Ubuntu, install BOINC and connect to BoincTasks

Install BOINC:

Click on "Ubuntu software" and search for BOINC, click on install next to BOINC Manager.
To add graphic drivers (CUDA/OpenCl) go to 'System Setting' and select 'Software and updates', next click on the tab 'Additional driver', wait and install any missing driver.

Allow remote access to the BOINC client:

Click on "Search your computer" , in the search field type 'terminal' and select the terminal icon.
In the terminal type 'sudo nautilus',  in the next screen click on '+ Other locations'.
Now select 'computer' and enter the folder 'var' ,'lib' and 'boinc-client' \var\lib\boinc-client.

Right click on the file 'remote_host.cfg', open with text editor and at the end of the file enter the IPv4 adres (or addresses each on a new line, without any spaces or comma) of your BoincTasks computer. It is best to setup a fixed IP address on the computer running BoincTasks.
Now open 'gui_rpc_auth.cfg' and enter a password. So this is a file that only contains the password, or is empty when you don't use a password.
In the upper right hand corner, click on the two arrow (network) icon, next click on 'Connection information', write down the IPv4 adress and the MAC (Hardware Address).

Restart the BOINC client:

For the changes to take effert, restart the computer or the BOINC client.
Go to the 'Terminal' and type /etc/init.d/boinc-client restart
The BOINC client is restarting and the changes will take effect.

Connect to BoincTasks:

In BoincTasks, select 'Computer->Find computers', the address is the start of the adress range of the IPv4 of the remote computer.
Click Scan address range, the remote computer should show up, if it doesn't check, your firewall.

Select the checkbox left of the IP and MAC of the remote computer and click on 'Add selected'.
If the IP address of the remote computer isn't fixed or you don't know, click on the IP Address and make it blank, so it leaves the MAC Address. The MAC Address will be used to find the IP Address of the remote computer.
You know the IP address and the remote computer IP address is static, leave the IP Address in the field, this connects slightly faster than with the MAC Address.
If needed fill in the Password or make sure to leave it blank.

Checking GPU usage:

Click on "Ubuntu software" and search for 'psensor' if the usage is less then 100% you may consider running multiple BOINC clients.

Running multiple copies of the BOINC client: