I think the BuildShare mod ( https://www.nexusmods.com/valheim/mods/5 ) can let you get buildings made by other ppl if they share it.

If that's a no go, I simply recommend starting with a 45 degree roof piece on the ground, add another to the top of it, put the wall pieces on the side, and just keep going as high as you want, and then do roof pieces down the other side til it hits the ground. So you'll have one side of the building done. (Think of a big triangle when you look at it.)

Then just extend the bottom roof pieces along the ground to make up the length of your building until you're happy. Then just close it up. It won't look pretty, but it's very simple.

If you wanna expand it more, you can dig alongside the bottom roof pieces on the inside and attach walls below them to make two side (pit) areas for storage or whatever.