Planning before sewing is probably a good thing, but leaving room for adjustment is more important. I have acquired the habit of buying less fabric than the pattern calls for when I have a specific pattern in mind (I'm short and can do that), and more than is needed for generic things. For instance, 1 1/2 yards is enough for a skirt or pair of shorts for folks up to size 14. I bought 4 yards of a pant weight that I didn't really care for, figuring I'd make a couple pairs of shorts alike. The price was right, and mucking about in the garden would ruin most fabrics.
I bought fabrics intending to use them in the next 18 months, and lo and behold, it was 18 years before I used some of them. I acquired an unhealthy fear of pants patterns. Not wanting to waste fabric, I held new patterns up against clothing that fit and could not understand how anything that shape could turn into the shape of the finished garment. I tested 6 different sizes of similarly designed patterns up against each other and found no discernible difference in size. I concluded that none would fit.
The solution is simple, but took me years to arrive at: avoid tailored pants. I should make pajama pants, yoga pants, pants with elastic waists. When there is oodles of fabric at the sides, the crotch length is less important. I avoided this for many years. Elastic is wrapped in polyester, and I was sensitive to polyester, particularly at the waist. Again, the solution is simple: make the waist larger.
This year, I bought Simplicity 0328 more for its t-shirt than its elastic waist shorts, and traced the shorts pattern a size larger than the pattern indicated. I added to back crotch length and subtracted from the front. When I got them to the try-on point, I noticed that the bits of fabric I had left for a waistband would have to be pieced and that the irregular stripe would not match. I looked at the 2 pairs of shorts I had made from the same fabric last year, and commenced deconstructing. Tedious, and it took 3 days to get enough thread bits out of the fabric. I had the fabric I needed, it matched perfectly, and I felt vindicated.
I did not use the Simplicity waistband with its cute sash ties. Sash ties would add several thicknesses of fabric at the waist, not to be considered in the current heatwave. I made a casing to hold the elastic, a folded ring. I put a ring of elastic inside, folded the fabric down, and sewed. Every so often I pulled on the elastic to get it past a ruffle of fabric. Then I sewed the encased elastic to the shorts.
In this way, I avoided having to thread elastic through a tunnel, a process I find to be hard on my finger tips and fraught with opportunities for disaster. If I used bodkins, I had to make the tunnel wider than I wanted just so the bodkin would go through. Sometimes I had to unpick seams because the tip of the bodkin got hung in them. If I used safety pins, there was always the chance that the pushing would cause the pin to open. I have been stabbed by open safety pins, I have had to open the tunnels. If there was a way for threading things through casings to go wrong, I managed it. And of course I assumed that all failures were my own fault, as opposed to normal differences in metals,plastics, elastic, fabrics, and humans.
In sewing, willingness to make adjustments is the key to success. At least, for me it is.
My sewing experience is that the only thing that matters is to be sure to cut everything too big, slam the thing together, and adjust, adjust, adjust. The garment is finished when I am tired of adjusting.