DC2 was born just before DC1’s second birthday. Tips I read, and used, focused around relating to DC1 and acknowledging how it feels, and ensuring they don’t feel ‘replaced’.
E.g., we had some special toys and videos that DC1 only got to play with/watch during feeding DC2, so it became something to look forward to rather than feeling like the attention is taken away.
When DC2 cried, we would say to DC1, “He’s very noisy!” and acknowledge how disturbing that was for him, but say e.g. “Baby doesn’t know how to ask for milk yet, so he’s noisy, we’ll give him some milk and then he might feel better” etc. When DC2 started crawling he would follow DC1, which DC1 initially found annoying, so again acknowledge that, “You just want some space, that’s ok I’ll play with DC2, but it’s fun to play with DC2, you can play with us too” etc.
Also getting DC1 involved in baby tasks, like bathing, changing nappies - not actually giving him any responsibility of course! - but asking him to ‘help’.
DC1 got used to it, and once DC2 started toddling and engaging more they started to hang out together. They now share a bedroom, play, DC1 tried to teach DC2 things... (5 and 3 now).