I have been running a Pathfinder campaign for two years and have used plenty of 3e and 3.5e adventures in it as well as native Pathfinder ones. (I've used multiple adventures from the Atlas Games Penumbra series, Green Ronin Freeport series, Goodman Games Wicked Fantasy Factory series, and older Paizo 3.5e APs and Pathfinder Modules.)
The short form is that 3e monsters/encounters count as CR (or EL)-2, and 3.5e ones count as CR (or EL)-1. In other words, you need a CR 8 3e monster or NPC to provide the punch a CR 6 one does in Pathfinder.
Besides that, you can do as much conversion as you want, but you really don't have to do any. Just run adventures higher in the range band - "D&D 3e for levels 3-5" means use on level 3 characters max unless you want to do conversion work.
My general process when using old ed adventures and actually converting is:
Convert major NPCs. This is where you get the largest differential, and you want your "bosses" to shine; it's not just hit points and attack bonuses but the cool new feats and classes and whatnot you want them to benefit from. I usually use Hero Lab to do NPC builds. (Many of the 3.5e Paizo APs have had fan conversions to Pathfinder posted on the Paizo forums.) My level 4 PCs can dogpile the heck out of a level 8 NPC boss if they're not done up well.
Sub in monsters where they exist in PF. Simple. Just look them up in the Bestiary(/2/3) and use those stats instead.
For lesser NPCs and monsters, either use tougher ones, more of them, or give them a quickie advanced template (or heck, just boost their hit points). For lesser NPCs, if you have the NPC Guide or Gamemastery Guide, there are a bunch of great NPC builds of various levels you can use for normal guards/bandits/soldiers/pirates/townsfolk/etc. and reskin as needed.
And that's it! I don't have the patience to do more, and unless you're publishing it there's no reason to.
This has come up on the Paizo boards before but as far as I am aware no one has finished their work.
Short answer is there's no easy way to do it, but you should be able to take the suggestions in the Conversion Guide for converting the core classes and apply it to the generic classes as presented on the d20 SRD.
The generic modifications (Hit Die, saves, etc) are going to be fairly trivial, the hard part is going to be selecting which class features you'd allow as "feats". If you are interested in going that route, I would suggest you start with some of the abilities from Fighter, Monk, Sorcerer (from the "default" bloodline), and Cleric and add some of the class features from those. But there's no "correct" way to do that. You also need to remember that Pathfinder has a tendency to ramp the power levels of all classes, bonus feats, class abilities, etc, so that needs to play a factor in your conversion.
Alternatively, and I almost prefer this suggestion, is to take the NPC classes presented in the Pathfinder Core book, and modify them to have PC BAB, Saves, and other class features. So, for example you'd be playing an Adept with a higher BAB, and better Saves (probably following the Cleric Progression), but with Adept abilities and spell lists. You'd have to expand their abilities further on if you plan on playing a long time like that...
So, yeah, no easy answer, hopefully this will help give you some ideas.
Best Answer
Refer to the DMG.
Chapter 4 of the DMG has lots of tables and advice for creating NPCs both as party followers and and villains.
Focus on fidelity of character and power level over slavish devotion to matching details.
Statistical and racial bonuses are going to be very different between the two. You should aim to keep the power level of the NPCs roughly the same in comparison to the party vs. trying to exactly emulate the spells and magic items the NPCs may or may not have had.
Use WOTC's own monsters as an example.
The Monster Manual has a lot of NPC spell casters and/or monsters that have strong innate abilities on par with spell casters. You may be simply able to reskin one of these or if not use them and their CR as a guideline for what should be level appropriate for the party.