2 Corinthians 4:13 - "We have the same spirit of faith as he (the psalmist) who wrote, 'I have believed and therefore have spoken.' We too believe, and therefor we speak.
To speak by faith is simply to speak words that agree with the word of God instead of the circumstances around you. It's speaking from your spirit instead of from your mind.
The bible tells us that God uses words to "call those things which be not as though they were." (Romans 4:17)
I speak the end result of what God's word tells me about my situation as opposed to the way it appears to the senses.
I stop "telling it like it is" and start speaking and believing the promises of God.
I put the power of God's word to work for me.
Proverbs 18:21 - "Death and life are in the power of the tongue: and they that love it shall eat the fruit therof."