I had to do this recently for one of my clients so i figured I would share it.
Creating a simple Paypal button on your website that allows the buyer or payee to enter their own amount
1. Login to your Paypal account
2. Click merchant services
3. Create a new button
4. The type of button is "Donations"
5. Click Save
6. Click the email tab
7. copy the link
8. Inserted as a link on your site
TIP: you can also create/upload your own image and add the link to your image
Negatives: their is no way to change some of the text that says donations, into something else for example invoice or payment
