The best way to ensure that everything in your model is coplanar where it needs to be, is to draw it that way. Daft as it sounds, while you’re drawing,make sure you use the inferencing engine: If you take the line tool and draw a vertical line, it will be blue… keep it blue. Then if you draw another line away from it on another axis, keep it on that axis… Then when you draw a line downwards again and you want it to be both vertical and the exact same length as the other vertical line, start drawing it so it’s blue, hold down SHIFT to lock it onto the blue direction… and then move your mouse so the cursor snaps to the bottom node of the first line.
Youre now guaranteed that these lines are the same length. If you push/pull the face you’ll eventually create by doing this, then its lines will be parallel. If you then want to draw another set of faces coming away from this (imagine we are putting the walls around the garden), you know that these faces were drawn vertical, and any pushpulls will be perpendicular, and the base and the wall faces will be co planar
If you’ve drawn two garden walls for either side of your plot and when you link the bottom corners, a face won’t form for the garden, then the walls are not co planar at the bottom.
Select the small bottom line of the wall, get the move tool and grab the selected line at the corner. Move it in the blue direction, hold shift, then point to the other wall, in the corner… I’ll do a video to show you what I mean
In the vid I draw 2 garden walls then make one of them deliberately wonky - obviously a face will not form… In sketchup a face won’t form even if the wall is 0.1mm wonky, and a simple slip of the mouse when you’re drawing, can put something off plane
So what we do is identify the points we need to be on a plane, and move them, aligning them relative to some other point that we know is on the plane we want to have… We end up with a wall that is still wonky in so many other planes, but they can all be fixed in the same way, or once you have the base of the wall sorted out, delete the rest of the wall and pull it up from the ground again