Uh...you don't? I mean, it's NOT UP TO YOU TO SAVE THEM, simply put. Your question is like someone picking up a 1000pg mystery novel and the first page says "Synopsis: The butler did it, in the Kitchen, with the Candlestick. Enjoy!".
So, you do nothing. The Players will figure it out...or not. Either way, it's not your problem and certainly not your "job" to help them not die.
Ok, I think you're doing it wrong. Y'see...when you're sitting there doing the DM thing, explaining the situation in terms of what the PC's would sense (see, hear, smell, taste, touch), all that information is already "letting your Players know". If they don't recognize the danger, then either they are really not picking up on it for whatever reason...or you just need more practice describing stuff. Keep at it! You'll get there!
Tips: Tone of your voice and "pacing". These, imnsho, you just have to learn/develop. They can't be taught. There is a distinct difference between "The chamber is 40' square, with a vaulted ceiling, 20' or so overhead at it's central point in the middle of the room. The walls have faded frescos of Sir Lightheart slaying various foul creatures...a black dragon being decapitated, a demon being struck down by Sir Lighthearts legendary sword, 'Angle-Kiss', and another of the Evil Ursurper, King Rendwood kneeling in defeat before the knight. In the north west corner is an old Romanesque style couch and footstool. Upon the couch rests the bones of some humanoid, once dressed in fine clothes, now rotted and eaten away. A dust covered golden goblet lays on it's side, just out of reach of the skeletons outstretched hand. Slowly, to your horror, red-glowing embers start to form in it's dark eye-sockets!". ... ...and... ... "The 40' square, vaulted room has frescos on the wall, but your attention is drawn immediately to the skeleton on the small couch in the NW corner...as it's eye-sockets suddenly spark to un-life with a hellish red glow!".
That is an excellent description of a scene. The problem is, it can also be used to describe a CR 1/2 skeleton or a CR 1 wight as well as a CR21 lich. Every skeletal undead is described as having glowing eyes.
The length of the introduction is detailed descriptive text, evocative and interesting... and giving no indication at all that the encounter will be out of their pay grade.
Unless of course you train your players to associate long descriptions with peril. I’m not sure that’s an association I’m comfortable making. Sometimes a long description may denote importance, or a busy scene. Not impending disaster.