You start with the positions one NEEDS.
Most leagues, you NEED 1 qb, 2 or 3 RB, 2 or 3 WR, 1 TE.
If you're in a snake draft, there is no upside to drafting a kicker before the last round. There's very little upside to drafting a D/ST before the last couple of rounds.
If you're in an auction, D/ST and K generally go at the $1 rounds. At that point, it's similar to just calling out draft picks.
Once the season is going, let's assume your league starts 1 QB, 2 RB, a RB/WR flex, 2 WR, a TE, K and D/ST.
Do you have a "stud" QB? An Aaron Rodgers, Tom Brady, Rivers (I know, this year he's been rough) etc? If so then priority falls to other positions.
Did you draft Vick? If so, you need an additional QB.
Then you look at your RB. Do you have 2 people you can definitely start every week?
Same for WR.
You will fill your flex spot out with value that either falls to you inappropriately (Ryan Mathews in the 5th or 6th round this year would be a good example) or with positional availability.
TE used to be tougher. There is a ton of value out there this year, and I don't think any of us knew it would be this available. So a lot of teams reached for Gates, Clark, Witten, etc... next year I know I don't do that.
After you have fielded QB, RB RB WR WR FLEX TE, then you spend the next rounds building depth based on what is available, or based on the flaws to the players you drafted. If you got SJax or RunDMC, you probably spent your flex on another good RB, expecting injuries. If you ended up with Vincent Jackson and Roddy White, you will probably bench 2 more WR and spend the rest on RB depth.
But you gauge need based on what you have.