Big congratulations to you
I have just had my DD2 (10 wo now) there are 20 months between DD1 and DD2. I am not gonna lie and say it is always easy with this kind of age gap but it has been easier than I expected it to be IYSWIM.
The biggest thing I can say is not to expect too much, in that don't try to do too much, make sure you have lots of lovely things for your eldest to watch on TV, toys etc. It's not going to hurt to watch a bit more TV than they normally do. DH and I got DD1 a painting kit and let her run riot with it so that I can concentrate on DD2 while DH is at work.
Involve you DD as much as you can with helping care for your lo, they are better at it than you think at this age. DD1 helps me to change DD2's nappy and does most of it, I just have to check she has cleaned her bottom well, she even applies her nappy cream lol (messy but good to watch)
I invested in a decent sling (moby wrap) to carry DD2 in while around the house so that I can still be hands on with DD1. It has been invaluable to me, DD2 loves it, it helps to settle her and eases her colic, she sleeps well in it and I can get on with things and interact with DD1.
If you can put all of your lo's bits (nappies, wipes etc) in a basket/box within reach of of your eldest and ask them to bring you what you need.
I try to get DD1 to interact with DD2 by asking her to tell DD2 what is in the book DD1 is reading or what is happening on the TV etc. DD1 loves it and is constantly talking to DD2 telling her stories and what not.
Hope this helps.
If I remember anything else I will come back.