I don’t have experience yet with this myself, but over the years these are some suggestions I have heard from others and books/internet:
Have special gifts prepared for baby [name_f]Dolores[/name_f]’ arrival. Tell [name_m]Harry[/name_m] that these gifts are specifically from his sister [name_f]Dolores[/name_f]. It can be good to have one for him wherever he will be during the birth (with your partner or family or sitter elsewhere), another gift for a few minutes after he meets his sister, and one for when you arrive home with his sister (or in the case of a homebirth, a day or two afterward once the excitement has sort of off). These should be little things that will provide some on-going entertainment, like a coloring/activity book or some Legos or something.
Before [name_m]Harry[/name_m] meets his sister, make sure he is well-rested. For example, if grandparents are taking him to the zoo or whatever and you deliver that morning, make sure grandparents put him down for a solid nap (even if he doesnt usually nap anymore) before meeting you at the delivery location later, or wait until the following morning. You’re trying to make sure he’s in the best possible spirits for the big moment.
When your son comes to meet baby sister, have your arms and hands free. Have someone else hold baby sister so that you can embrace your son with a big juicy hug and lots of kisses. This will help make it clear how happy you are to see him in particular and will make it clear that he is still just as special to you as your new arrival. If you are in a hospital bed or otherwise incapacitated during this time, have him join you/snuggle next to you in the bed or sit on your lap if you’re in a wheelchair and are able to (if you don’t have a c-section wound or extreme vaginal discomfort that would be hurt by more pressure). Then have your helper bring baby sister to you and big brother and you can hold her together. You should try to talk to him now about how to act around babies (“gentle hands, soft voices,” etc.) But have the helper around just in case this experience becomes too overwhelming for [name_m]Harry[/name_m] or [name_f]Dolores[/name_f] so that someone can tend to [name_f]Dolores[/name_f] right away… someone who isn’t you. So that you can still assure [name_m]Harry[/name_m] that you are there for him and you can talk through his feelings right away.
During the following weeks, try to set up a routine of what to do when [name_f]Dolores[/name_f] needs your attention and [name_m]Harry[/name_m] wants your attention at the same time. If you can set expectations for what happens when [name_f]Dolores[/name_f] cries hard and you need to tend to her, it can avoid a lot of jealousy in the moment. Like, “when [name_f]Dolores[/name_f] cries hard, that’s when mommy would like you to be a helper and play quietly with your trains. As soon as mommy is able, she will come play with you, too!” And then follow through on whatever your routine is, trying your darndest not to leave [name_m]Harry[/name_m] longer than a few minutes while settling baby [name_f]Dolores[/name_f]. As much or as little as he is interested, have [name_m]Harry[/name_m] help you with “big brother” tasks like getting a clean diaper or onesie from a drawer. If he is not interested, don’t make him help out in this way, but if he is, he will likely gain pride from feeling like he’s really helping his little sister and being a good big brother that way.
[name_f]Hope[/name_f] that helps! Looking forward to seeing your announcement and knowing what middle name you pick! Congratulations!