Give them lots of hugs and tell them you love them.
I know it’s hard to give them time but when you can, try and give them some special time alone.

No hard and fast answers as every child is so different. Definitely try to include them with the baby and have some dedicated special time with your first born.

Include them at all times so they don’t feel left out!

All i can suggest it to have some special time with just you and the older child once the younger on is napping,, Also see if there are some activities that only they can help with due to your other kid been to young.. It can be something as simple as helping prepare tea or folding the washing.. My kids job is to tell what pile the clothes go into as i fold them.. Its funny but it makes them happy and feel needed..