It can be a simple add, especially with an existing tool like Paypal or Stripe. For WordPress there is a very richly featured plug in called GiveWP that allows you to create a donor wall, send follow up emails, etc. It does require some time to get familiar with the tools and configure it, but makes it pretty doable for a beginner or intermediate web manager.
If you are using third-party systems like Raisers Edge, it can add development time and complexity because the developer will have to figure out how to get data entered from a website form into the database. It’s possible to embed a form from a third-party software company, but in my experience, those are not very friendly to mobile phone website visitors. So, if possible I would allocate the time and budget for a developer to set it up for you to make it mobile-friendly and easy to use.