You actually already have part of the answer: to make it easy to type, use only lowercase letters.

Now the trick is to know how many letters to use. If you want, e.g., 40 bits of entropy (a quite high goal for a password), then you must accumulate at least x letters such that 26^x > 2⁴⁰ (i.e. 9 letters). 9 letters are sufficient if they are chosen uniformly, randomly and independently from each other; do not try to do it in your head, the human brain is a terrible RNG. Instead, use a good RNG (coin flips, dice, a computer with /dev/urandom...) and, that's the important bit, stick to the result: you get your 9 letters, accept them and learn them.

If you want to have a password that you can "easily" memorize then you need more letters. For instance, you could generate 100 sequences of 10 letters, and keep the one which pleases you most; since 26¹⁰ > 100*2⁴⁰, you still get your 40 bits of entropy.

See this question (in the crypto-specific stackexchange site) for details on the involved mathematics.