I would pre-wash the fabrics and then starch with whatever you prefer (Starch/Best Press/whatever).
To mark, I would use a very light, thin line chalk pencil if the fabrics are dark, or a very light touch with a water removeable blue pen if the fabrics are light. The blue pen comes out with just a touch of water. I usually keep a damp cloth handy to dab away the lines as soon as I don't need it anymore. I soft brush will brush away the chalk lines. Jamie Wallen uses a chalk pencil to mark called a Boehing or something like that. I think he also has a video on marking utensils.
Being careful not to quilt lines close to each other in a back and forth motion will help keep the loft smooth between lines.
Don't wash, obviously.
Can't help with washing the batting. If the item is never going to be washed, I don't see that it matters.

I hope we get to see your creation.

Watson

I cannot for the life of me work out how to post an image...

Many of the national quilt show quilt makers use Misty Fuse for stabilizer. Especially for the very clean smooth modern quilts. When I went to all the national show before covid, I would wait by the winning quilts for some of the makers to show up. Usually most of them were there to stand by their quilts several times a day.

They like all quilters love to talk about their quilts. I learned one of the national winning quilts used Walmart fabric in part of the quilt. One used a projector to mark her quilting design, one wadded her quilt up and tossed it in the corner for months before deciding to finish. I ask questions.
I asked one what brand of sewing machine did she use. Her answer was very surprising., her fancy big machine broke on a Friday night. She was on a deadline so went to Walmart and bought an off the shelf Brother. She said don't tell anyone but I still use that Brother and the broken machine is still in the closet. She said I'll use the cheap machine until I kill it and then buy another. Cheaper then one service fee on the fancy one. There is always a lull of no one around but you and the quilter and that is the time to find out all kinds of neat things about the quilt and the quilter..

Ha, that's amazing! I realised the other day that the more serious I get about the quilts (eg burying threads rather than locking) the less I use my the fancy stuff on my janome MC8200... and I've even considered getting an industrial (one with speed control) to get a more perfect straight stitch, because I don't do any free motion at all...

I'm not sure if the attachment I posted is actually visible to anyone, but if so you can see that it's all straight line quilting, none of it very dense at all. I was reasonably pleased with the smoothness of this one but a recent one was much more crinkly - neither was prewashed but the crinkly one may have been post-washed at a slightly higher temperature. I foolishly failed to keep notes!

There are some people that recommend doing all straight line quilting in one direction only.

Even with a walking foot, there is some drag on the fabric - and if it is all dragging in one direction, the result will be a lot smoother than when one alternates the stitching directions.

http://quiltingdigest.com/6-great-ti...hine-quilting/

Yes, when there are lots in the one orientation, for eg the horizontal lines in the quilt I posted, I usually do half in one direction and flip, try and avoid zigzagging where possible.
I wonder if Misty fuse or similar is the answer... it feels like I'm basically asking linen to stop behaving like linen... !

There was a post a while back about the method Sharon Schamber uses to wash her quilts: spray washing. Her quilt certainly appears to retain that “not-crinkled” look after washing. More here: https://www.quiltingboard.com/links-...g-t316088.html

Wow that Sharon Schamber video is amazing! Looks like lots of fun (though perhaps less easy in rainy UK). That quilt looked as thought it had some trapunto but you're right that the rest of the 'normal' quilting definitely looked very smooth.

Maybe I should use two layers of batting!