I am also on this journey, the corners for me are the challenge I have Ina wrights book on Will and do the mitring thing I do fil the joints and then use a jewellers fret saw to re-establish the horizontal mortar joints but its very slow. I have tried laser brick cut kits but these also fall down on the corners. I have attached a couple of goes at this a standard will craftsman pub and a scratch built  signal box with lcut.co.uk windows and wills for the brickwork and windows

Many thanks for posting your picture and thoughts, @deepfat.  I must say your corners still look good to me, and I like the finishes on the white-painted pub and signal-box colours.

I have ordered the Wills kits book from the library, and will have a read.  I amended a Ratio line-side hut kit recently to try my brick skills, replacing the stone courses with brick and adding a chimney.  Sadly, I still can not cut and file the brick sheets to a 45 deg. mitre, so again am filling in with modelling clay to 'sculpt'.  I wish I could construct some sort of template or jig to ensure I am cutting and filing at 45 deg., but can not think how.

@C126 Your are very kind, but looking at my uploads I should have gone to specsavers! I have a huge file to do my mitring and that helps as it's heavy and I can run the section along it too make sure the edge is square, but the angle is by eye and I figure if I over do it the edges will meet OK and then use those wills fillets to reinforce the corners but I'd love a gizmo too . I use a battery operated dremel with a cutting disc too but that starts to melt the material.

These are looking really good. I struggled with corners when on my first structure, the station building on Bovey Tor. I tried various ways but always had issues with the corners. I'm not sure the brick courses are that accurate on the polystyrene sheets. 

Anyway after all that, I decided to try the 'Pendon' method. And personally, found it much easier. And because you're working on piece of card there is only one join and that's at the back where it can't ever be seen.

George