Gentlemen,
If he is trying to duplicate a factory load, he'll likely need a chronograph to establish the velocity his factory ammo has out of his particular rifle.
To the OP,
Beyond that, (like mentioned) read your manual. Select a powder based on the load data in the manual, where the starting and max loads (for that bullet weight) bracket your desired end state velocity. Then work up a load. Also, understand, factory loads do not typically use "cannister grade" (stuff people buy in stores) gun powders. They use industry supplied bulk powders which are then blended to achieve a desired velocity and pressure. So, the home reloader can not create an exact duplicate most times, but they can develop one that has similar pressure and velocity, which will yield (typically) the same results as factory bought ammunition. There are exceptions to this rule, but it is rare when factory ammo uses cannister grade powders (308FGMM and early 6.5CM ammo come to mind as ones that used cannister grade powders).