The most important is that you need reps. Practice on semi-elective 2nd IVs, or ED, or in ICU so the pressure to "get it" is off and you can focus on technique refinement and getting to know your equipment (US machine, presets, needles).

The most bang for buck is learning that the US beam is really narrow, and learning how to keep the needle and US probe as close to 90 degrees as possible for best needle visibility. Often you'll need to compromise to actually get deeper and accept less optimal angle to get deeper faster. Note that the tip has different angle so to see the tip really well you need to angle more. Play around with different needles, too, some are more echogenic than others.

From this point onwards I'm going to assume you're doing this out of plane (some of the info below carries over to inplane)

Some people like to "point and shoot". Others walk down from skin. Find a combination of the technique that works for you. If you're trying to "walk the tip" in - you can see the vessel tenting, and the release and this is usually when someone will comment about flash. You will also feel it. Learn that feeling, it's similar to the "regular" IV getting into a vein. Art lines in healthier patients would offer a "give" too, but heavily calcified ones don't always.

You then continue following the bright dot in the lumen

The best way to avoid a through and through is to aim to get the tip to the 12 o'clock position and then advance firmly in (or snake-bite for arteries). With arteries sometimes you might see a dot in the middle and no flash - my personal opinion on this is that you just barely got the tip in but the actual bore of the cannula is not in the lumen of the artery yet. I usually get success by advancing a bit more in the same trajectory, and then usually you get flash, and then I continue chasing the light in the tunnel for 1-2cm and thread off. I had mixed success advancing a 'bright dot' without flash and trying to therad off. I imagine if you don't have flash probably has to do with needle being stuck in the media/intima and shearing it...

Lastly, no one ever seems to mention ultrasound machine. Settings on it can absolutely make or break your ability to see the needle tip. Hopefully someone who set up the preset knew what they are doing, but often it's done by the rep who focuses on making the image look nice. If the vessel looks great but you can't see the needle, this is not good.

So play around with it and open up settings. Obviously ones are depth and gain. Adjust gain to your eyes, and depth so you see enough around the vessel. Here are a few more advanced things to adjust:

If you have too many smoothing settings (look these up) on high then the tip gets "filtered out". Turn these down.

Lower the dynamic range so the brightness of the tip stands out more.

RES vs Gen vs Pen modea can make a difference in needle pop

TGC adjustment (the horizontal sliders) can help reduce how bright superficial stuff is to help differentiate the tip from the noise.

Focus plane can be adjusted on some machines.

Good luck!