Understanding how to construct Bruhat-Tits buildings for non-split groups by Galois descent

There is a forthcoming book "Descent in buildings" by Bernhard Mühlherr, Holger Petersson and Richard Weiss, which should appear soon (published by Princeton University Press). See http://press.princeton.edu/titles/10649.html. It addresses exactly this issue, from a very building-theoretical point of view.

Jiu-Kang Yu has an article "Bruhat-Tits Theory and Buildings" which has appeared in "Ottawa Lectures on Admissible Representations of Reductive $p$-adic Groups". It doesn't directly address your question, instead it contains a guide to the literature on Bruhat-Tits buildings.

There is a 26 page pdf version of Yu's article. The published version contains a bit more information, but not a lot more (a few more references are added, plus a short introduction, plus the typesetting is updated). Unfortunately the printed version is hard to find.