The printed grainlines and markings on a pattern piece are meant to help orient your fabric so you don’t have a twisted, wonky garment, pieces that won’t match up, or just a confusing mess of fabric once the pattern paper is gone. Two markings that I’ve seen especially help a new sewer with this are the notches and grainlines.
The grainline of the pattern piece is supposed to be parallel to the selvage of the fabric. Most beginner patterns have a long arrow for this, and it’s the easiest way to get that long arrow parallel to the selvage. But when it’s not parallel, the results are all over the map. Your skirt may twist, your sleeves may seem to pull, or your facings may not lie flat. This doesn’t necessarily have to do with your stitch line at all. There could be an issue that arose at the cutting table.
When positioning a pattern piece, measure from the grainline to the selvage on both ends. Adjust it until both distances are equal. Pin or weigh the pieces down only when you have ensured these are equal. It may feel slow to measure both ends before cutting out, but measuring your pattern piece helps train your eye to see the grain. Additionally, it helps you treat your fabric differently than you would any other paper you can flip upside down to make better use of the space.
Notches do a slightly different job. These marks are supposed to indicate which fabric pieces should meet each other. A notch along a sleeve may match one along an armhole; another notch along a waistband may indicate which part of the band goes along the side seams. When these marks are ignored, you may think the two fabric pieces will line up okay when sewn, but the result will still look wonky. The fabric may stretch in one area, not curve at all, or the ends don’t line up at the seam.
Before you remove your pattern from the fabric, ensure that you have notched the fabric pieces to correspond with the pattern marks. Some sewers make little snips inside the seam allowances of their fabric, whereas others use tailor’s chalk or a fabric marker to do so. For the beginning seamstress, this depends on which mark is easiest for you to see, and which mark will not ruin your fabric if you don’t use it right away. Your mark doesn’t need to be big; it just needs to be there for as long as you need for it to ensure you’re lining pieces up right, so you know which parts match up with which parts when you pin your fabric pieces and sew along those lines.
Try it out with some scrap fabric. Try out a few notched patterns that are easy and make one with the fabric grainline marked, one with the pieces positioned correctly along the grain and the other slightly off. Lay one pattern piece on top of another. Press both and see what is different. Take the notched pieces and pin them together. You don’t need to make them wearable. The point is to see these small markings in action to convert a random piece of fabric into a planned, purposeful construction.
When your garment doesn’t look how you want it to or doesn’t fit right, don’t just blame your machine or your stitch. Check if you actually measured your grainline, marked your notches along all of your pieces, and whether you pinned or clipped the pieces together before sewing those lines. It’s hard to notice, but these simple little tasks are the ones that can make all the difference in whether your next seam feels clear or not.