Filling out contracts when wholesaling

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I am meeting with a seller tomorrow to look and walk through a rental property that he is very motivated to sell. If we can come to a offer This could be my very first deal wholesaling, but I am nervous about filling out the sales and purchase agreement with the seller when we come to a offer. I don't want him to think I'm clueless and don't know what I am doing. I am in Colorado and have reviewed and read through the 17 page contract to buy and sell real estate (residential) but am confused on what all I have to review with the seller and what I do and don't fill out. Do I fill out what I know already before I sit down and sign with seller? Do O get a attorney to review the contract before or after the contract is signed? To make sure I use the right verbiage and it is correct? Sorry I am very new and do not want to screw up with a legal contract.