To ask what for advice on becoming a grandparent

My advice is that as her mum, you are the one best placed to know your DDs unique personality, the relationship she has with you, and where she'd see you on the spectrum of welcome support to overstepping. So rather than seek a blue print of advice to be the perfect GP, centre your DD and your SiL in your behaviour and try to be the GP they would want you to be. Which will be different from what people on here would want.
The only rules I'd say, is let them lead. If asked an opinion, give it honestly (if they wanted text book they'd Google, they're asking you for a reason!) but gently and without judgement.
Try very hard, particularly in the early days after the birth to remember that however excited you are and how in love with new DGC you are, your DD is YOUR child, so should remain the focus of your attention. My mum was the only person who visited me after the birth who looked at me, kissed me and asked after me before looking at DD. It meant a lot and I made sure to do the same for my DD.
Which I guess sums up my approach. My children are my children and my role as GP is primarily to support them. In the process I've developed a lovely relationship with DGC in their own right, naturally.

I don't have children but have read a lot on here about issues between grandparents and their children/grandchildren. I also had a quite distant relationship with my mother.

I'm going to say "well done" for seeking advice on this, because I've seen so many threads about GP over-stepping OR not being interested enough (apparently)!

Ask her what she needs from you and give it, no questions asked.

This - a few people who had not been particularly close to us beforehand (their choice) suddenly wanted a very involved relationship when DC came along. We were perfectly happy for them to have a relationship with DC but it's very difficult to go from visits/interactions once in a blue moon to the expected much closer contact. Also quite offensive - the implication is that you weren't worth the effort before DC.

Really try to make sure you have as solid a relationship as possible with the couple and show you are there for them, not just the baby, and it's likely to all be much smoother.

Don't take full advice on here. I applied what I was reading and my DD told me that she didn't understand why I was being distant.
Ask about her, not just the pregnancy.

I can't stand pregnancy symptoms being minimised, the woman is growing a whole new human, that's tiring.
Give support as a couple.
Decide what, if any financial help you are going to give. My Mum bought the pram system for my DD's first, but my DD chose it. I bought the second pram and both times gave gift vouchers for Next and Asda. Don't give with strings attached.
Just listen, give opinions when asked.
She needs to feel supported, don't commit to anything you might not be able to do.
Keep the pregnancy about her, not just the baby coming, or, as said, women can feel like incubators.
Respect their rules and boundaries. Unless something is dangerous, or your DD is being badly treated, don't give unasked for opinions, or negative feedback.

Find moments to praise her. I found when I was a mum everyone was full of helpful advice but no one ever said how well I was doing or said anything positive at all really. They weren’t being unpleasant and I was doing fine - they were all just desperate to share their knowledge and lost sight of how it would feel to have nothing but a slew of constant suggestions

Ok so deal with repairing the core of the relationship between you and your daughter first before anything else

Expect parenting advice to have changed eg sleeping on back vs front. No water to newborns. Paced bottle feeding.

dont give unsolicited advice - they will come to you if they want help

What is your relationship like?