I am now doing that. Seems to me there are two distinct skill sets that come together here:
1. The skill of sewing and stitching as such
2. The skill of garment construction - sewing and stitching then is used to attach the constructed pieces together.
My approach has been to work on 1. first by making household items as I need them using salvaged/second hand/curbside-find material. Because I am working with material cut from shirts, t-shirts and sweatshirts other people threw away, it's not such a big deal if I fuck it up. In addition, most household items I have found myself needing are simple iterations on attaching rectangles to each other. I have made 3 pillows cases, a laptop sleeve, a coin purse, a headphone pouch, and a water bottle sleeve which have all turned out serviceable. My most unfortunate project has been a pair of house slippers which turned out rather sad. Fortunately, they are vaguely slipper-shaped, in the sense that even though they're kind of misshapen and came out different sizes even though they were cut from the same pattern, one can stick one's feet in them and putter around the house, so I call that a success.
Skills I am developing so far while working on 1.:
1.1. Practicing different stitches. Acquiring a sense of which is useful for what.
1.2. Practicing basics of construction: how would it come out if you attach 2 pieces right side together? wrong side together? When should you work from the front, when should you work from the back and turn? What shape of fabric should be cut if you want the fabric to form the top bulging part of a slipper? etc.
1.3: unintended consequence: my relationship to time is changing. But that is the subject of another post. I am developing the patience needed to stay with a piece of work. This patience is essential if you want your piece to be crafted well, but it turns out patience doesn't mean what I thought it meant in salaryman world. I used to think of patience as putting up with something dull and tedious and tolerating the irritation until done. Turns out it means crafting without regards to time: it takes as long as it takes, and if it becomes dull or tedious, you can stop. There is no deadline. You do not need to be "efficient" or account yourself to anyone.
My personal approach has been to watch many youtube tutorials of the same thing. Each person makes the thing in roughly the same way but somewhat differently. I internatize the general principles of how the thing is constructed and why, and then I freestyle on my own. This suits my INFP-ness much more than picking one tutorial and precisely following a set of steps. A different approach might work for you depending.
I recommend sticking to Indian, Thai, South American, etc. tutorials even if they are not in English (the people show what they are doing very clearly, so language is rarely an obstacle. Often under one of those videos that was made all in Thai you would see thanks from Russia, Indonesia, and Israel
This makes me v happy). The reason for choosing tutorials from developing countries is that they rely less on assembling pre-polished pieces purchased at Corporate Crafts. Many US tutorials are ads for overpriced craft products in disguise. Tutorials from developing countries, on the other hand, rely much more on skill and using what is on hand. An American tutorial would say, "use bla weight fusible fleece, link in description." An Indian tutorial would say, "well I am using some stuffing here, but if you don't have any you can use 3 layers of old towel." American tutorial: "here are my soles I purchased". Indian tutorial: so we will iron a plastic bag inbetween these 2 sturdy pieces of fabric until they fuse together and we will cut out the bottom of our slippers from that. Thai/South Korean/JP tutorials focus much more on the intricacies of a particular skill.
Bonus: I find tutorials rather claming. Good for my anxiously depressed ass.
As for tailoring down specific pieces, there are two approaches that I see:
2.1: easy simple fixes: e.g.
-taking in a pair of trousers from behind to make the waistband smaller without necessarily changing the rest of the garment.
-Making your jeans more fitted to your legs by making a second seam a little to the inside.
-Shortening trousers: cut off, re-hem. Also used for converting trousers (jeans or track pants) to shorts.
-taking in a men's shirt on the sides so it's a bit more fitted to your body. Also involves making a second seam closer to the body but gets a bit more involved under the arm hole. Also, this is obviously only a useful strategy if the shirt is short-sleeved. If long-sleeved, you might fuck up the length of original sleeves etc.
2.1 is the next step for me personally. Approach is the same: watching many many tutorials until I feel brave enough to butcher a second hand store piece. These are all v useful alterations, but also only really work if the piece you are altering is already close to your size.
2.2. Thorough disassembly of original piece: part or entire. Re-cutting of original panels to size. Replacing some pieces of fabric as needed. Reassembling piece to fit.
This is currently over and above my head, but I find this dude quite inspirational:
How to recycle your jeans, parts 1-3:
https://www.youtube.com/watch?v=P2T30A-mn3E
https://www.youtube.com/watch?v=IMzu8i_LP8A
https://www.youtube.com/watch?v=nZySwsbYaSM
I am getting into "visible mending" and the japanese concept of boro: the repeated patching up of clothes as they wear out, thus making them warmer in the process. These days, boro seems to be made by "distressing" new fabric with a file, but original pieces that have been mended over generations are considered antiques and sell for ridiculous amounts of money.