I put my 7 month old and 2.5 year old together.
In a nutshell, we first put our younger one down then we did prayer and song with the older one. Before I carried the older one to bed, we would drill him with questions: What will you do when the baby cries--and he would answer: close my eyes and go to sleep. When may you get up--and he would answer: when the sun comes up.
It seems like I'm missing a question we would pose to him, but I can't remember it. Also, the answers he gave would provided to him by us.
To our surprise, he would just deal with it when she cried. A few times, very few, the younger one would cry incessantly and then we would just take the older one out (into our bed) until the younger one stopped crying (if she didn't have a dirty diaper and she had been nursed enough).
Good luck
Just be consistent, it will work