This doesn't seem like a very exciting tip but it is one of the most important tips I can ever give to budding decorators. There is a standard that is used by everyone in the industry.
When you are measuring anything, you measure the width first and then the height, and then the depth.
Windows - always measure the width first. You will need to know this when you order custom draperies
In a house...always measure a room clockwise.
For bedrooms you start with the master bedroom and then the next room to the right is bedroom #2, then #3, and so on.
I have made every mistake in the book and measuring wrong is costly.
Hope this comes in handy for you. I will add more industry standards later.