Well it depends on the app, as some will need additional configuration, such as enabling inputs. But to start simply untar them in $SPLUNK_HOME/etc/apps and restart splunk to make it available.
follow these instructions:
1. Log in to Splunk Web and navigate to Apps > Manage Apps. 2. Click install app from file. 3. upload an file, click under file and go search the app that you want install 4. click on upload after your restart splunk web
please forgive my english