That's easy - seriously. It's time consuming, and you have to stay very alert for it, but...
All you do is follow each step every single time.
*Make sure every case is trimmed to the same length, and sort those suckers by weight. Consistent cases.
*Hand seat all your primers. Go for the exact same "feel" or pressure to seat them, and don't overseat them. If one doesn't feel right, or takes more pressure to seat, that case goes into the "Range Ammo" pile. It's not used for precision loading.
*Measure every single powder drop. Every one.
*Seat every bullet to the same depth.
*Measure the OAL once loaded. If one is shorter, don't mix it in with the rest when you're going for accuracy on targets. It won't run the same, and will impact differently. If one is longer, no big deal - seat it deeper.
Literally, that's it, in a nutshell. When loading for precision, you want every single one that you make to be like all the others. It's about consistency.
Coming UP with the load is a different subject.