First of all, welcome to a really enjoyable part of the hobby

I've built a number of bikes, and I've never faced anything. I wouldn't worry about it.

Leave the steerer length to the very end. You'll need to make sure you have enough steerer for the spacers you want, plus the stem, and then add your 3mm so you can have a 5mm spacer on top, then the topcap.

If it's a new frame, those grommets are to plug holes if you're going to run wireless electric shifting, or the like. Once you get cables run, you'll see where they need to go/not go.

And google/youtube/bikeforums should get you through it all safely. If you haven't bought a torque wrench, do that before you tighten anything. Something like https://www.chainreactioncycles.com/us/en/x-tools-essential-torque-wrench-set/rp-prod155414 will work great, but to get the right torque on bottom bracket cups and cassette you should grab one like this too: https://www.harborfreight.com/hand-t...nch-63880.html

With carbon, respect the torque settings.