You could do it the same way as in the desktop versions. Draw a rectangle positioned at the miter angle and intersect faces. I assume the parts are components or groups so you’ll want to make the cutting plane inside the group or component. If you share the model it’ll be easier to show you.
bac a fleurs.skp (277.1 KB)
Thanks, here’s the model and I’m also about to try your suggestion.
Okay thanks! I see you flipped my components since they’re going to make copies, good idea. I’m going to have to play around with this for a bit - I just picked up Sketchup again for the first time in some years and things have changed since it’s online now. I’ll let you know how it goes and thanks again for your help 
Yes. Flipping the components creates the mirrored parts so the miters are in the right place. One thing you’ll need to do is make sure you get the components aligned correctly. They were a fair amount out of alignment. I fixed them up. You can take a look to see how things end up. I also centered the top pieces on the planter.
bac a fleurs.skp (209.1 KB)
Good luck.
There are still desktop versions available.
Great, I’ve fixed the alignment, flipped the components, but I’m having trouble with the intersect faces command. “with selection” isn’t an option that appears for me. Is this because I’m using free or am I doing something wrong?
Okay, not all the geometry was selected, maybe that’s it. I’m going to keep on trying, definitely much slower pace than you but that’s the pace of learning.
Keep plugging away. You’ll get it.