I donated blood for the first time last month. I had Soylent for supper the night before, Soylent for breakfast, and samosas for second breakfast right before my appointment. I had plenty of fluids(water, tea, almond milk), making sure to drink a bit more than usual starting the day before my appointment. After they were done taking blood, I felt no different from before I had walked into the clinic. It was like nothing had changed. But I still sat there for a while eating a bunch of free cookies and drank a couple of juice boxes. Took a few packs of cookies and a couple more juice boxes with me which I had throughout the rest of the day. Had Soylent for lunch and supper, and then I had steak because I ate supper too early. Had mostly Soylent and iron-rich foods for a couple weeks after before returning to my usual eats. During those couple weeks I felt cold all the time; not sure whether that was because of blood loss or because it was starting to get colder outside, but it’s much colder out now than it was then and my cold tolerance is back to normal so who knows.
But yeah, I don’t think it really matters too much what you eat, as long as you do eat plenty and make sure you’re well-hydrated. The day of the appointment, blood sugar is important so feel free to indulge in sugary snacks like I did. After that, just try to eat healthy It’s often recommended to get a lot of iron-rich foods in afterwards, but not everyone needs it. I was too lazy to get blood tests done to find out, so I did it anyway. Iron-rich food is mostly awesome stuff, anyway.