Haha yep LOTS of trial and errorThe steeper sides are harder to disguise, plus harder for anything to get out if it falls in (hedgehogs etc and even froglets may struggle). The only downside to gently sloping sides is that the pond takes up much more space than you initially think! On another pond in my front garden, I used hessian covered in a mix of soil and gravel to encourage moss on the sloping parts. I tried it on the steeper bits too, but it didn't work well there. You can buy coir matting pre-seeded with wild flowers but it's very pricey for what it is.
As for putting soil on the liner to plant marginals, the only thing to be careful of is that it doesn't dry out in summer when the water level drops. I've also done this on one side of my biggest pond; it first drops for marginals, then the edge is raised again then drops again for a bog area/overflow. Play around and experiment unless you have immense experience in pond building, there's often a lot of trial and error! At least there was for me (and still is!).
I basically built a little one last summer in the lovely weather, went and spent a fortune on pond plants then decided it wasn't big enough
So went and built another one on a spare bit of land we hand, my wife isn't amused ponds keep popping up everywhere haha
Wish I had found this site last year though!
There is a side that's gently sloped and I've watched frogs get out on that side
So I'll build up the one side that's really low, so I can increase the water level
For the side that's already sloped, I'll take a look to see what I can do
I like the idea of gravel to make a wee kind of beach, but it's how I stop the gravel from just sliding right into the water!