If the rest of the day does not make any sense without this part, I will try to write it in immediately, starting with writing that I forgot to mention something, and maybe summarizing in what sentence the correct order of things so that it stays in order in my head. If I need to or want to write it only later, I write a sticky note and put it on the next page so I don't forget to do that. I sometimes end up keeping the sticky notes and sticking them more permanently into the journal to decorate a bit.