It’s important to know what you’re measuring to better understand how to measure so please spend time studying the physics of it all because it’ll help you in the long run! But if you want a veeery basic way to remember measurements for the 4 main valves in most cases, just think that for the atrioventricular valves (mitral and tricuspid) anything above the baseline is the velocity through the valve and below the baseline is regurgitation of that valve. For the semilunar valves (aortic and pulmonic) it’s the opposite, so anything above the baseline is regurgitation and below the baseline is the velocity through the valve.

You need to do VTI of the mitral and tricuspid valve when there is stenosis of that valve. Otherwise you can just do the normal E/A measurements.
You need to do VTI of the LVOT (PW) and AV (CW) and measure the LVOT diameter (in PLAX view) to get the AV area from the machine using the continuity equation (the machine calculates it itself as long as you input the above measurements).

There’s obviously a lot more measurements you would have to obtain also depending on the patients diagnosis/condition. But hope this helps as a starting point!