You’re free to choose where your donate button should appear on your website. However, there are places that are more suitable than others. Not every visitor will study your site carefully, so a placement that’s relatively high up is a good choice. For example, the eye tends to quickly find the donate button if it’s placed in the top right corner, as it usually blends in well and doesn’t distract from the rest of the page. In principle, you can also place the button further down so that the call to action appears immediately after the content. However, not all readers will see it this way.
It’s also recommended that you place the donate button not only on your homepage, but also on various sub-pages. Here, too, you should consider where placement makes sense. However, if the donate button blends well with the site’s content, there is nothing to be said against using it more frequently.