Prerequisites
To setup a free linux instance on Oracle Cloud Infrastruture is necessary to have a Oracle Cloud Account, otherwise sign up for a Oracle Cloud Free Tier.
Toolbox
Networking
Login into Oracle Cloud Infrastructure portal and from the sandwich menu follow the path Networking > Virtual Cloud Networks.
Click on Start VCN Wizard button and then select Create VCN with Internet Connectivity.
Fill the VCN Name with something like oci-vcn-<virtual-cloud-network-name>, then click Next, finally Create.
Linux Instance
From the sandwich menu follow the path Compute > Istances and click Create istances.
Fill the Name with something like oci-instance-<instance-name>.
In the section Image and shape click Edit then Change image to select your preferred image.
Select Canonical Ubuntu for example, Always Free-eligible, and click Select image.
Leave the default shape VM.Standard.E2.1.Micro which is Always Free-eligigble.
Check if the Networking section displays the previously created virtual cloud netowkr.
In the Add SSH keys select the option Generate a key pair for me.
Click on Save Private Key to save the private key file to name something like oci-instance-<istance-name>-ssh-key-YYYY-MM-DD.key.
Click on Save Private Key to save the public key file to name something like oci-instance-<istance-name>-ssh-key-YYYY-MM-DD.key.pub.
Finally click Create.
SSH Private Key Conversion
Execute PuTTYgen and clcik Load button to load the private key oci-instance-<istance-name>-ssh-key-YYYY-MM-DD.key.
From menu bar follow the path Key > Parameters for saving key files... and select PPK file version: 2, then close the dialog and click Save private key to save the converted private key file to name something like oci-instance-<istance-name>-ssh-key-YYYY-MM-DD.ppk.
Setup SSH Connection
From the sandiwich menu of Oracle Cloud portal follow the path Compute > Istances, select the created instance and from Instance information section copy the Public IP address under the Instance access subsection.
Execute PuTTY and in the Session frame fill the Host Name (or IP address) with the Public IP address copied from the Oracle Cloud portal, leave the default Port: 22.
In the same Session frame fill the Saved Sessions with something like oci-instance-<istance-name>-putty.
Under the PuTTY frame path Connection > SSH > Auth, in the group Authentication parameters, click Browse and select oci-instance-<istance-name>-ssh-key-YYYY-MM-DD.ppk as Private key file for authentication.
Finally from Session frame of PuTTY click Save.
SSH Login
From PuTTY Session frame, select oci-instance-<istance-name>-putty, click Load and then Open.
From the sandiwich menu of Oracle Cloud portal follow the path Compute > Istances, select the created instance and from Instance information section copy the Username under the Instance access subsection.
In PuTTY console when promt login as: input the username copied and login.