While sewing garments isn't all that different from sewing quilts, there are a couple of key notions that make the process that much easier. I'll be talking through all the important ones, why they're helpful, and what you can substitute if you're not ready to go all in and purchase each one yet.
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Flexible tape measure
In all likelihood, you already have one of these on hand and have never used it. A hand-me-down from a parent, grandparent, or aunt.
A flexible tape measure, or tailor's ruler, is simply a long strip of material, usually vinyl that has inch and/or centimeter markings on it. You use it to take your body measurements in order to determine what base size of pattern to cut.
Taking your measurements is a critical step to get a garment to fit in a way you're happy with. You don't want to skip this step.
If you'd prefer not to purchase a flexible tape measure, you can use a spare scrap of non-stretchable fabric or string to take your measurements. Read out the number associated with them by holding the fabric or string against your standard quilting ruler.
28 mm rotary cutter
A 28 mm rotary cutter is just like your 45 mm rotary cutter, except smaller. It's ideal for getting around the curves found in garment patterns and makes it easy and simple to cut out your pattern pieces without having to pin your pattern pieces down or trace them.
You use it by holding it vertically and slowly cutting around your pattern piece.
If you don't have one already and don't want to purchase one, a sharp pair of fabric scissors or your standard 45 mm rotary cutter can be substituted, but you'll have way more control and a much better outcome if you use a 28 mm rotary cutter. It's also MUCH safer.
Pattern weights
Pattern weights are small, dense little pouches designed to be scattered over a paper pattern template to hold it in place while you cut around the shape. They're used instead of pins so you can cut your pieces out faster and more accurately.
If you don't want to purchase pattern weights, there are a variety of things you can use instead. Large washers (2" in diameter or so) from the hardware store are a great alternative. You can also use any heavy, spare object that you might have lying around in your sewing room. Cones of thread are a favorite of mine.
Wonder Clips
You probably already have these for binding quilts. If not, I would strongly recommend investing in some. Wonder Clips are perfect for a lot of garment sewing because it makes it easy to hold bulky garment fabrics and seams in place without distorting them like you might with pins.
If you don't have then and don't want to purchase them, pins are always available, but mini binder clips also make a great alternative. You can grab buckets of them super cheap around back-to-school time and use them throughout your sewing room for lots of different things.
Stiletto
If you haven't discovered the wonders of a sewing stiletto, you are missing out! This is easily one of the best tools I know for piecing and it comes equally in handy for garment sewing.
A stiletto is just a small, pointy tool that you can use to keep fabric layers in place as they go under your machine. Think of it like a mobile pin or a substitute finger that you don't have to put in harm's way in front of your needle. The best ones are metal with a comfortable handle. These are especially great for garment sewing because they can be used to hold curved seams, like sleeve seams, in place while sewing.
If you don't want to invest in a stiletto, a chopstick or tweezers are a great alternative.
Tailor's ham
A tailor's ham is a very funny-looking object. It's basically an oval, stiff bean bag that is used to press seams that are curved. Think armpit seams or necklines. You place the ham inside your garment and it provides a curved surface against which to press with your iron tip.
While you can usually maneuver your garment to keep small sections of a curved seam flat enough to press and slowly work you're way around, having a tailor's ham to provide a curved pressing surface certainly makes this process a lot easier.