When am converting monsters that were meant for CL 6 in 3.5 to a CL 6 in 5e......

[CyberThread]

What should I keep an eye on, so I don't create feather pillows for monsters, or something that can steamroll my players?

[Knaight]

I'd recommend starting from scratch. Take the core concept of the monster, figure out key mechanical implications, then build a 5e monster using the 5e guidelines. So, take the dragon turtle, for example.
Core Concept: Giant sea turtle that breathes steam and threatens boats.
Mechanical Features: Pretty good AC (see: turtle), strong and tough, steam breath, capsize ability, excellent swimming, stealth in the water, human intellect.

So, you then go to 5e. You pick an AC that is pretty good by 5e standards - the AC 25 of 3e for the dragon turtle is really a bit high by 5e standards, and direct conversion would cause problems. You pick strength and constitution scores that are pretty good by 5e standards, again the 27 and 21 are a bit high. You write up an ability for the steam breath, capsize, and stealth benefit, again using the 5e scale. Then get the numbers to work with the 5e guidelines.

Basically, there's no direct mechanical conversion. You don't take existing values and add or multiply things to them. You figure out what the mechanics and flavor initially represented, then figure out how to represent that with the new mechanics.

[TurboGhast]

Basically, there's no direct mechanical conversion. You don't take existing values and add or multiply things to them. You figure out what the mechanics and flavor initially represented, then figure out how to represent that with the new mechanics.

Agreed. Every edition of D&D does things differently, so most things can only be used RAW in one and must be converted to be used the others.