Add a Subscription Form to your Sidebar (Widget)
To add a subscription form to your website’s sidebars, simply do the following in your site’s Admin.
Go to Appearance > Widgets, like this:

Find the Wysija Subscription widget amongst all available widgets. Drag it to your sidebar or any widget holder:
Select the form you want to add in your widget:

Make sure you visit your own site and double verify that you can subscribe successfully.
Before version 2.4
Set the options you want. Make sure you Save when finished:

You can add as many widgets as you want. Read this article to add a form in a page or article.
