Reference a few reloading manuals and pick a powder recommended for the cartridge- I use 4064 with good results for most non-magnum cases with bullets over .22 caliber. Reload each cartridge the same way- do all of each operation such as resizing, trimming, measuring powder, or seating bullets at least in the same session, so that each cartridge is done consistantly (its ok to do most of the steps on separate days, but at least do each individual step itself at once)
Start with the minimum load, and work uop in .5 gr increments to the maximum load- I load 5 for each increment. Label the differently loaded cartridges carefully and keep a notebook or folder containing all of the reloading info- such as date, powder used, primers type, bullet type, overall cartridge length, trimmed case length, wheter or not the cartridge was crimped. Take the loads to the range and shoot them at seperate targets for each load. Watch for pressure signs everytime you go up a load increment. Take yoru time so that the barrel doesn't overheat and skew the results. Once you get find a load that gives you the best grouping, you can adjust it by a few tenths of a grain either way, though this doesn't always help. I seat my bullets out so that they fit in the magazine- most sporting rifles are going to have a fairly long throat which would making seating the bullets close to the lands impractical.

If you want to switch to a different powder heres a little shortcut that should steer you in the right direction that I learned from an Army Ballistician which usually gets you pretty close using the Lyman manual. Using the listed minumim and maximum charges and velocities listed in your manual for a particular powder, plot your velocities on the vertical axis and the powder charges on the horizontal axis. Leave plenty of spaces between max and min figures. put a dot at where the axis' meet for the min vel and min charge, and the max vel and max charge. Draw a line connecting the dots. Do the same thing with the powder you want to switch to. Put an x on the line representing your first powder where you got your best accuracy, then find the powder charge to the left or right of this x that will give you the same velocity for the second powder. As long as the powder charge is within safe range given by the manual, yoiu should be able to work loads near this powder charge for the new powder and find the rifle's sweet spot pretty quickly. If you have a chronograph, the whole process becomes much more precise and easier.

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