grainline and pattern placement help anyone? Sewing Machines Parts and Sales in Los Angeles

I am a beginner but I am already pretty comfortable at my machine. I’ve sewen a few garments myself now, mostly deconstructing and reconstructing old clothes of mine in to new ones that fit better to my liking. I am about to take on an ambitious project of recreating (with a couple simplifying modifications) my FAVORITE flowy oversized long sleeved shirt. I feel comfortable tracing and creating the paper pattern but when it comes to placing the pattern on the fabric I am a little lost. I know what the grainline is, parallel to the selvage, and if it is cut against the grain the fabric will hang weird. So say I’m ready to put the pattern to the fabric, how do I know which direction the grainline should be for each piece of the pattern?
I don’t even know if I’m making any sense haha. Thanks for your help!

Learning how to sew when you know absolutely nothing can be a little intimidating and overwhelming…..