Congratulations! I’m sure you will be a great MIL because you are asking how to be!
I agree with not giving advice or interfering.
Make sure you still show an interest in your DIL and your son when you message, speak or see each other rather than just locking onto the baby like an Exocet missile.
Keep any early visits to them short and sweet and offer to bring specific food with you like sandwiches from a favourite deli or something you make you know DIL enjoys.
Only say positive things about the baby. New parents are hyper-alert to “subtle” hints about how often the baby feeds/sleeps/cries/burps etc. The same with comparisons to your own children when they were babies. No knackered parent with a contact sleeper wants to hear that yours were swaddled and slept in their own Moses basket from coming home from hospital.
Check what their wishes are re posting on social media. Don’t post photos of the baby without checking if they are okay with it and if you are posting about the birth with their permission, don’t just cut them out of the story. “Congratulations to Gemma and Oliver on the birth of their beautiful son! Thrilled to be a grandma!” is much nicer than a photo of you and baby with “New grandson!” or similar.
Try not to brood or fret if a visit is ever postponed or cancelled because of a bad night or DIL not being up to it, reassure them it’s fine. It does happen, particularly in the early weeks when baby blues or exhaustion can just wipe you out.
Don’t get sucked into competitive grandparenting with your DIL’s parents or your own friends. Just because your mate has her 6 weeks old granddaughter for overnights doesn’t mean you’re less than if you don’t. The baby sure as hell won’t remember it.
If your son complains to you, be very wary about saying anything that could be interpreted as critical of your DIL. Err on reassuring him that things are tough for both of them in the early months and it gets better. There will be times when either or both if them get ratty and the last thing you need is your son telling your DIL in a row that “Even mum thinks you’re unreasonable and she had to look after us without my dad lifting a finger during the week…”.
The surest way to build trust is to follow their preferences on care to the letter and not to query them. They’ll both feel very protective and a relative saying their babies slept on their side and never came to any harm does not inspire confidence in their willingness to do otherwise if caring for another baby.