first install jenkins
sudo pacman -S jenkins
then start it (or enable it first)
systemctl start jenkins
lookup the initial password. (show the systemd log files for user jenkins)
sudo cat /var/lib/jenkins/secrets/initialAdminPassword
open the dashboard view http://localhost:8090 in your browser. Do not create a new user as suggested, it will not work. But search for the login button and use the long password you found in the previous logs.
Now you can create a new admin user.
[update: 2016-12-01]
The admin key is not printed in the logs at startup anymore but i keep this as a reference how to retrieve the systemd logs for a process that runs as a specific user.
journalctl -u jenkins
here you find the similar lines
...
May 17 11:15:46 hwm-arch jenkins[8295]: *************************************************************
May 17 11:15:46 hwm-arch jenkins[8295]: *************************************************************
May 17 11:15:46 hwm-arch jenkins[8295]: *************************************************************
May 17 11:15:46 hwm-arch jenkins[8295]: Jenkins initial setup is required. An admin user has been created and a password generated.
May 17 11:15:46 hwm-arch jenkins[8295]: Please use the following password to proceed to installation:
May 17 11:15:46 hwm-arch jenkins[8295]: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
May 17 11:15:46 hwm-arch jenkins[8295]: This may also be found at: /var/lib/jenkins/secrets/initialAdminPassword
May 17 11:15:46 hwm-arch jenkins[8295]: *************************************************************
May 17 11:15:46 hwm-arch jenkins[8295]: *************************************************************
May 17 11:15:46 hwm-arch jenkins[8295]: *************************************************************
...