How to properly apply the AES Mixcolums?
Do you mean where we come up with 0001 1011 (also known as 0x1B)?  That's actually mentioned in the answer:


  Multiplying by $2$ is what your question is about: it is equivalent to shifting the number left by one, and then exclusiving-or'ing the value 0x1B if the high bit had been one (where, in case you're wondering, the value 0x1B came from the field representation)


That is, the $GF(2^8)$ representation that AES uses is a polynomial representation, based on the irreducible polynomial $x^8 + x^4 + x^3 + x + 1$; 0001 1011 (or 0x1B) are the lower 8 bits of that polynomial.

If you don't understand that, well, just assume that 0x1B is a magic value that just happens to make things work...