Install Patch (only for PRO Variant Users)

Download and Install Journal3 Patch (as per your OpenCart version), after following the installation steps mentioned in the previous articles.

You can find the patch from your license section.

More Setup for Journal3 Template (for all variants)

Disable: Journal > Product Extras > Extra Button for Notify Stock (if added)

Enable: Journal > Styles > Product Page > Add to Cart >  Out of stock visibility > Enable 

Optional CSS Setup

If you see the notify button is not aligned properly on product pages, try the following

Add the following style to the class .notify-button 

width:50%; 

(Optional) Add code to common.js file

Open catalog/view/theme/journal3/js/common.js

Add the following as shown in the screenshot below.

if (json['hberror']){
if (json['hberror']['oosn']) {
notifypop(json['hberror']['oosn']);
return false;
}
}

For OpenCart 2.3.x.x and Journal3 Users

Go to Admin : Extensions > Extensions > Select Themes from drop-down > Edit Default Store Theme. Choose journal3 for theme directory.