Sewing - absolute - Basics

This may seem overly idiotically simplistic. Two recent sewing projects of mine have failed because of some very basic things that I overlooked.

The Fabric
Most fabrics are woven. The terms for the threads going in different directions are "weft" and "warp"
Cutting fabric
In terms of cutting a fabric, I get the idea of cutting "on the bias" - that is diagonal to the threads (both the warp and the weft).
It's ordinary cutting that I'm not so clear about. For example, should the waistline of my garment be parallel to the weft or to the warp? Or if the question is easier phrased thus: "should the waistline be perpindular to the weft or to the warp?" It seems so simple, so basic, but I'm stumped.

(And this is to say nothing of the fabrics inside and out-side.)

Another term - "selvage" - that's the edges as seen on the right and left in the images above.

Window Dressing
I did pick up a nice tip about cutting a fabric "on the straight of the grain". Make a snip through the selvage, pull a weft thread and it will cause a puckering effect through the weft line of the fabric (if loosely woven enough). The fabric can then be cut squarely along this line. So certainly with window dressing, I am confident that the fabric must hang as it was woven, eg the top of the window is parallel to the weft.*

Dress-making
Now for dressmaking... aha! often the pattern will have arrows# - these arrows indicate the direction of the grain (direction of the warp, me-presumes). Therefore... I will take it that my skirt is to hang like a curtain and its waistband is to be perpendicular to the warp, parallel to the weft. I understand this now. Will it still make sense if I need to re-teach myself six months down the line?

*This tip was picked up on: www.seweasywindows.com
#Learnt on www.startsewing.co.uk