You can either make a separate video for the preroll ads, you just use your best video. What you choose is up to you, sometimes I actually use my channel trailer for my preroll ad! They are quite effective as long as you target correctly and set the bid to 0.01. DO NOT let youtube randomly show your ad to people, OR set a price for you.Tell me more please about your experiences with preroll ads. How long are your ads? Is it similar to a channel trailer?