In the user dir create a folder

sudo mkdir ~/.ssh
sudo chmod 700 ~/.ssh
sudo vim ~/.ssh/authorized_keys

paste the content of the public key

sudo chmod 600 ~/.ssh/authorized_keys

In putty configuration -> Connection -> SSH -> Auth -> credentials
Voeg daar je *.ppk file toe

indien de folder niet op naam staat 

sudo chown -R user:user /home/user/.ssh
sudo chown -R user:user /home/user/.ssh/authorized_keys

save the session