You can start with bre-built buildings in the meadows. Repair them, put a fire outside with a basic roof. You could claim an entire farm and use those buildings. The downside is that found buildings are often too small to be very useful, including and especially the stone towers.

For a basic house, stick to rectangle designs. If the floor is an even number across, the slanted roof pieces will meet in the middle. If the floor is an odd number across, you will need the inverted V piece to go down the middle. For some reason those pieces don't snap evenly to the other roof pieces, so I always put a wood beam at the end corners up high and that provides a place for the middle piece to snap cleanly to the other roof pieces. Once you have one in place, the others will line up with it. You can then remove the wood beam if you want. You will end up with an open triangle on both sides, which you can fill with triangle and wall pieces, although those sometimes take a few tries to snap correctly. You could also just call it a window, or put X pieces or beams in there.

The corner roof pieces are trickier to snap into place, so if you have trouble building skip those. I'd also recommend keeping to a single story building, because if you go tall you'll be dealing with supports, and if you're wrapping around a tree or a rock the level changes will be a challenge to cover. Go the viking way and go long, maybe double walls tall and not too wide. Do a fully roofed sample slice to check the height before you waste time on a whole building you can't support. Core wood works great for support framing, but make sure it's touching the ground and not just a floor. If it's blue it's on the ground.

For a chimney, a simple design is a three-sided wall box facing in and then a second three sided box on top facing out. If you want it taller, put a full square layer in between. Then stick a slanted roof on that facing outward. Smoke needs a full square of room to vent properly.

You will need to see the edge of the piece you want to snap to. This is especially difficult when expanding floors up high. Build temporary ladders and platforms to give yourself a better view.