The orginal question was regarding the quickest and easiest way to add a PayPal Donation button to the WordPress Sidebar instead of directly in a WordPress post.

To add a PayPal donation button to your WordPress Sidebar you would use the Text Widget. This is the method you use for a WordPress.com website. For a WordPress.org website you can use this method or several other possible methods to add a PayPal donate button to the WordPress Sidebar.  Using the Text Widget method to add the PayPal donation button to the Sidebar is by far the quickest and easiest method.

Click on your Appearance panel, click Widgets, drag the Text Widget to your Sidebar and copy and paste the PayPal donate button code into the Text Widget window.

This additional info is for anyone who wants to know how to create the PayPal donate button and get the PayPal donate button code.

Create a free PayPal account. I recommend choosing the Premier PayPal account, but a Personal PayPal account is ok. You can always upgrade or downgrade later on.

Log into your PayPal account and click the Merchant Services tab. You will see a Create Buttons section. Click on Donate (button type). Follow the simple steps to create the PayPal button (button code). At the end of the process you will end up with the PayPal Donate button code, which you can copy and paste to your website.

If you make a mistake or you want to change the PayPal donate button code at a later time you can just click on the Merchant Services tab, click on My Saved Buttons link under the Create buttons section to go directly to your saved buttons that you created.