The objective of this tutorial is to be able to create a good vocal sound.
To sing well and properly, you must first follow these steps about posture, air flow etc.
It doesn't much matter whether you are standing or sitting as long as your back is straight. If you are slouching then it is like when you're at a resteraunt drinking a malt and the straw kinks and you can't get it unkinked. The same thing happens to your air flow when you are slouching. If you sit up straight however, the air is allowed again to flow properly. You may choose standing or sitting according to what will allow better posture.
When you take in air, make sure that your shoulders do not move. Your stomach should expand due to your taking in air using your diaphragm. If you don't use your diaphragm, you will run out of air faster then you would using your diaphragm. Your diaphragm should be your best friend while singing.
You absolutely have to sit up straight! You cannot be slouching if you want a good sound to be your voice. You will run out of air VERY quickly if you do slouch. Just like the malt example, if you slouch, air will not come properly. Slouching is the biggest NO when singing.
After reading, watch the videos.
If you have any questions feel free to ask! :)