Hiya!
One? Ok...
"Don't build encounters/adventures to the PC's specifics".
What that means is don't refrain from putting in flying monsters if nobody in the party can fly or has missile weapons...or don't refrain from putting in a magical locked door that requires a Knock (or similar) spell to bypass simply because the party is all fighters and thieves.
It's NOT your job as DM to "cater the world to fit the PC's strengths and weaknesses". You can, but from a players perspective (mine, at least), it feels like I'm being babysitted (babysat? babysit?... ? ). Your job is to present a believable fantasy world/setting in a consistent manner that lets the Players play in it "believably", without meta-gaming. If the Ogre Forest has ogres in it, don't suddenly only have random encounters in it be limited to "goblins, kobolds and giant rats"...because later when the PC's are higher level and suddenly encounter nothing but multiple ogres, trolls and giants in the same forest...well, you just shot your credibility as a consistent DM and totally obliterated any believabiliy of your world; congratulations, you're now running a Computer RPG but without the cool visual graphics and sound.
To put it another way...it's not your job to keep the PC's alive. Let the players do that. If they want their 1st level group of 3 to travel straight through Ogre Forest, even when others warn them not to because of, well, all the ogres, let the chips fall where they may. Not your problem.
That's my advice. "There are 5 PC's of about 6th level"...that's the ENTIRETY of what I need to know when writing an adventure or dungeon-delve for them. Their PC's races, skills, spells, equipment, etc....don't care. The world they live in doesn't care...why should I?
^_^
Paul L. Ming