Have not played for years, but I used to base my decks on the 3rd methord using two colours. 3rd land, 3rd creatures and 3rd spells. Remember that low powered creatures can be beafed up by spells.
He's talking about the board game, not the Card Game. They share some similarities, but deckbuilding is based on colors of your plainswalker character and a set point value max that each card contributes to.
Step 1 is to buy at least 2 copies of each set so you have extra cards to work with. Otherwise you will have to take apart certain characters decks in order to play other characters. This is especially true with the multi-color plainswalkers from the second and third sets.
I'd have to look back through and see what I set up for mine, but ideally you want to keep an eye on the total points costs for the decks and the armies and keep them relatively even and they should be balanced for the most part.