In the first step, go to "System preferences ..." (1)

Then, in the "Internet and wireless connections" section select "Network" (2)

Add a new network connection by clicking on "+" (3)

From the tab "Interface" (4) select "VPN" (5)

From the tab "Type VPN" select "L2TP over IPsec" (6)

Enter the name of the connection, e.g. "VPN (L2TP)" and click "Create" (7)

In the field: "Server address" (8) - enter the name one of the VPNonline servers
In the field: "Account name" (9) - enter your login
The list of available VPN servers can be found in the client panel: https://portal.vpnonline.pl
Click "Authentication settings ..." (10)

In the "User Authentication" field, select Password
In the field: "Password" (11) - enter your password
In the "Machine Authentication" field, select "Shared Secret"
In the field: "Shared Secret" (12) - enter: vpnonline.pl
Click "OK"

Check "Show VPN status in the menu bar" (13) and click "Advanced..." (14)

In the "Options", select "Send all traffic over a VPN connection" (15)
Click "OK"

To confirm all introduced changes, click "Apply"
To connect to the VPN server, click "Connect" (16)

After the correct VPN connection is established, the status will change to "Connected" (17) 

To disconnect the VPN connection, click on the upper bar and from the drop down list, select "Disconnect VPN (L2TP)" (18)

In the first step, download the application Tunnelblick (OpenVPN)

After downloading  go to the installation, open the Tunnelblick application (1)

Double-click the Tunnelblick icon (2) to start the installation (3)

We will be asked to enter the login (4) and password (5) of the local user with administrator privileges, click "OK" (6)

After installation, we will be asked: "Do we have configuration files?"
We choose, "I have configuration files" (7)

Then, click "OK" (8)

Download the configuration profiles: Tunnelblick Profiles
Unpack and open the downloaded folder (9) with connection profiles (10)

Tunnelblick has been installed, located on the top bar (11)
Click on the icon and expand the bar, select "VPN Properties" (12)

We have 2 new windows open: Folder with profiles (13) and Tunnelblick Configuration (14)
We are moving connection profiles (13) to the Tunnelblick program (14)

We will be asked whether to install new connection profiles only for me or for all users.
Select "Apply for all" (15) and choose "Only for Me" (16)

We will be asked to enter the login and password (17) of the local user with administrator privileges, click "OK" (18)

After installing new connection profiles, select one of them (19) and click "Connect" (20)

We will be asked to login (21) and Password (22) from our VPN account.
After entering your Login and Password, click "OK" (23)

If the data is correct, the VPN connection is established and the configuration is downloaded (24)

The connection has been established correctly, we can check it by clicking the application icon (25)