Typically you line up the timing marks before you take the pump off. On the Roosa master IP there is a little rectangle cover you pull off on the side of the pump, and you turn the motor over until the little dash marks line up. Since he bought it apart, he will have to find the timing mark on the motor usually an indicator by the crank pulley, turn it to 0 and make sure its not 180 degrees off then line up the dashes on the pump and slide it back in the gear. Sometimes it takes a try or two to get the gear on the right tooth.
One more thing that is important, if it has the same style pump as ours did, the shaft pulls out of the pump and there are to seals on it. Its is VERY important they do do not get folded or damaged or else you will have a motor full of diesel fuel.