My first objective after I have chosen my components is to find multiple sources of load data for the cartridge I'm loading. Some powder, bullet, cartridge combos I load for don't exist, so you have to have a good idea of burn rates, case capacities, and use other data to get you in the ballpark. If you're loading for something common, then find at least 2-3 sources of published load data for the bullet weight and powder you intend to use. Take the maximum powder charge of all three, and figure out the average max charge weight, say it's 50 grains. I always start about 3gr under their max charge, usually somewhere a little above minimum.
Once you have a starting and end point established, I load one round each from where I started to maximum. I load in 0.2 or 0.3gr increments all the way up to max. I then shoot each round over the chronograph, recording each shot in my log book and waiting 1-2 minutes before shooting another round. Once this is completed, I go back over my notes and look for charge weights with minimal changes in velocity. Say 48.5gr was 2850 and 48.7gr was 2848 and 48.9 was 2855fps. This is what's called a "node" or flat spot in velocity. Once you find this, load 5-10 rounds up in the middle of the node, so 48.7gr in my example, and then go chronograph those loads. This will verify you're shooting in a node. You're looking for an extreme spread ideally under 20FPS and standard deviation under 10FPS. Once you find this, it's time to move on to bullet seating depth.
Before even shooting a round, I find my lands with the bullet I intend to use. If I'm not limited by a magazine, I'll start 0.010 off the lands and do all of my velocity testing there. After I have my velocity node figured out, I'll test seating depth. I start with course adjustments of 0.010 and work away from the lands. I shoot 3 each from 0.010 off to 0.050 off. From there I'll pick out what grouped the best. Let's say 0.030 off shot the best. If you're satisfied with accuracy, then load your ammo at 0.030 off and be a happy camper. If you're note satisfied, but 0.030 was still better than the others, seat bullets at 0.025 off and 0.035 off and test them. If you're still not satisfied, pick which seat length was best, say 0.035 off, and load ammo at 0.033 off and 0.037 off the lands. Go test these. If you still aren't satisfied, try a different bullet or powder.