You begin with a piece of poster paper. All I had was a small piece so I improvised. I made two small ones but I didn't like that idea and it was taking too much time. So I cut pieces from the second one and stapled them to the first. It's not a work of art but it works for me.
Second, gather a pile of magazines and go through with a pair of scissors and a glue stick. Cut out what you love, what jumps off the page and paste it to your poster - words, pictures, colors etc.
Put it up somewhere that you will see it and watch what happens. It's a lot of fun and a visual reminder of what direction you want to head in.