Scenario: you’re using opensource software and you aren’t donating yet, however you have enough money left over every month to do so. Something(s) is holding you back from donating. Think about what that is and what would have to change for you to change that.
I donate to a number of free-software projects via https://liberapay.com/ which lets me control the individual donation amounts and frequencies, and pay from a single account that I top up periodically.
When approached about allowing me to donate to them via Liberapay, I’ve had several people respond positively and set up their end of the deal. It often just takes a friendly conversation.