The method of adding a donation button will depend on what you want to do.

If you have created a donation page on a third party service (such as JustGiving etc) that you want to link to - then you can create a standard link to this page and style it as a button.

You could add this into the Raw Banner HTML box in the theme options, if you wanted the button at the top of all your pages. Something like this HTML would do it:

<a class="btn btn-primary" href="https://link.to/donation/page">Donate</a>

Another option would be to create a page on your site with all the donation information on it, and create a button to link to it in the page header (as above but the URL links to the Donations page on your site).

Some third party services let you create a custom button/widget and will give you some HTML or JavaScript to embed on your site. Donorbox can do this, and there seems to be a way to do this with PayPal here.

To do it this way, first you should create the button/widget that you want using your chosen service. Once this is done you can then either embed their code directly into the Raw Banner HTML (or Raw Footer HTML) in the Theme Options to put it directly on every page, or you could embed it onto a Donations page on your site (using the Raw HTML includelet) and then link to that page.

Sorry that I'm being a bit vague with the answer here - though I can't really be super specific because the "best" option will depend on what you can generate using your chosen third party tool, and whether you want to create a Donations page on your site, link to a donations page elsewhere, or embed a donate widget onto every page of your site.

If you want a more tailored solution for your site and a specific donation service, then you can contact admin@e-voice.org.uk with further details and I may be able to help.

Thanks

Joe