I've been playing this on the hd collection for a few hours now and man I don't know what it is but I cannot get the hang of the stealth, especially where I am now.  I am at the part where I need to kill 8 enemies so EVA can escape, and every single time I am spotted within the first 2 minutes.  I'm constantly using the cameo to blend as best I can and even when its at 90% they still seem to see me.  I tend to try and use the CQC as much as possible to save the silencer and ammo.  Any tips or tricks would be awesome.

I just opened the latch to go under the ground floor and tranquillised them in the shins.

don't worry, That's the hardest part in the game for me, besides a part were you have to assume someone else identity to infiltrate a complex. Most of the game it almost seems unfair to the guards, but that's what makes 3 so much fun.

Step one is don't rush it. Stealth games are about patience and timing. After that it's all practice. Also in MGS3, enemies will hear you coming up behind them unless you're stalking. To do that, use the directional buttons, NOT the thumbstick.