What should be considered when building a natural stone wall made of slate?

Yes, I also find the question of the shape crucial for the approach. I am a theorist myself, so not well suited to quickly come up with a secret tip. I could imagine that you would do yourself a favor by first building a kind of hybrid between teaching and a mortar sled; with "freestyle" stones, that could be a helpful third hand, and then generally proceed layer by layer. I would probably first "pre-layout" the next layer dry, placed next to it. Mortars, as far as I know, mainly differ in how much water the respective stone absorbs during setting. Regarding the foundations, I could imagine starting with a layer of formwork stones and embedding the first natural stone layer into the still fresh concrete. You probably come across my favorite sentence starter from cookbooks: "beforehand, we have ..."
Do you have low walls where you can already practice?

So the stones are completely differently shaped. But I would say like in the picture I posted. Some are a bit more elongated, some a bit rounder, but as is the case with slate, they are all pretty flat and relatively even. Walls that I can climb over, I will already have beforehand. However, I still don't have certainty that the mortar will withstand frost. I assume the hardware store will give me the right stuff that is good for natural stones and frost-resistant. Can someone say if I can take a day off in between or if I have to build continuously because it has to be wet-on-wet?

I suppose the hardware store will give me the right stuff that is good for natural stones and frost-resistant.

Opinions among the learned about the qualifications of sales advisor performers in hardware stores vary greatly. But manufacturers also have hotlines.

Can someone say whether I can take a day off in between or if I have to build continuously because it has to be wet-on-wet?

Even with normal masonry, you stop work at the end of the day, whether the floor slab can go on it or not. The last layer is simply covered so it is rainproof. If I were you, I would probably take a break after an emptied food container.

Thank you very much in advance for your answers!

I have now read that trass mortar is apparently the remedy of choice for natural stones. However, I can also omit the steel rods in the middle of the wall, as they do not seem to be compatible with trass mortar.

Which cement should I use so that the wall doesn’t crumble apart again next winter?

I have slate in the garden... a few knee-high walls, like raised beds... delivered by us, placed by the landscape gardener. Mortar or cement: for God's sake, not with slate. Natural slate is stacked, but not mortared!

I would like to clarify the question about the format - do you have completely different sizes or already split processed? I would do the latter without mortar. It is not child's play.