Carry a sketchbook with you and sketch in it everyday without exception. In a year or two you might have a few good quality pieces to start a portfolio. Besides this, Master students are expected to know how buildings are assembled through each step of their design so best start to learn these through elective courses.

The easiest way to communicate with the schools you hope to apply to and seek guidance from the faculty.  Only that school's staff will be able to help you with the quality and content of your admittance portfolio.

Atkin,

There is no reason why you should not include your civil engineering work, especially if they involve sketches of various structures, in a portfolio. Why? because you have the advantage of proving your designs visually, graphically AND mathematically. I'm an architecture student. Believe me, the ability to prove that your designs work is a very big deal. Atkin, finish your degree in civil engineering, become certified (EIT to PE), then pursue a degree in architecture if you want. The one question that I'm learning to ask myself in terms of design is "why am I doing what I'm doing?" If you can answer that question, you will definately answer any other design questions. Hope that helps.

I don't understand the need for a 'project statement' or 'questions' to work on...

If you are going to apply for a masters program, you will be going in to a curriculum in which nobody has a background in architecture, therefore, the people applying don't have "buildings" to show in their portfolio.  

Do you explore any other creative outlets?  art, painting, drawing, photography?  That is most of the other people applying to a similar masters program will be showing in their portfolios....If you try to fake a bunch of uninformed buildings for a portfolio now that is going to be very evident.  Stick to what you know and you will learn about buildings when you start your masters program.  I think the key is just to show some creative aptitude.

I did a Civil undergrad then Arch grad - took some elective courses in architecture (history, drafting, and a couple of studios -they let me do them as electives).  For the portfolio I took a couple of drawing classes and had some hobby photography; the architecture faculty and grad students were kind enough to evaluate and critique my portfolio before I sent them out.  If there is an architecture program there it may be helpful to ask.

I will say that both in school and in the practice they are very different approaches to building, but if you get high enough up in the fields they work closer together (from engineering think Tomasetti, Nordenson, Arup, Happold, Schlaich, etc).