I really can't stress enough how important it is to do test runs and samples when trying new products or techniques. Doing a test run on a sample piece may be the difference between an enjoyable hassle free experience, or severe costume malfunctions during a convention or outing.
The same could be said for trying on and wearing your costume before an event. Perhaps you followed the pattern or instructions precisely. But this won't always guarantee the comfort or fitting of a costume. It could be something as simple as a hemline not sitting how you would like it. Or various seems that rub your skin. Now, I for one am one of those people that tries a costume on the second the last stitch has been made. Pretty sure that's something we can all relate to. But things like manoeuvring around easily, up and down stairs, or getting through doorways (for larger armoured outfits) just a few things you may want to practice before heading to a convention and feeling like a fool while trying to figure out how you will manoeuvre through the door gracefully!
As cosplayers we tend to tolerate a lot when it comes to looking your absolute best at a convention, but sometimes the discomfort can be very much avoidable.
But lets get back to sample pieces. I've found this particularly important when painting. You mix the paint and get it just the exact shade you want it, but come back once it's dry and it's now a different shade. Or perhaps that colour doesn't show through enough on that piece of fabric you want to paint, or that piece of armour.
Shading and highlights are a tricky one to master when painting a costume. It's easy to feel like you have gone overboard when you're applying the paint or product. But once dried the difference gradients may be hardly noticeable.
Also, when using a new product for the first time it's best to have a go on a small test piece. Or perhaps it's a product you're very familiar with, but applying it in a different manner than what you may usually use it.
I've used liquid latex on many occasions, but recently I used it as a primer for a foam suit of armour. My foam armour had many small pieces attached together with hot glue, and I was worried for the integrity of the glue should the weather become warm and the glue become 'melty'.
So with my usually product, I trialled this new method on a small piece of foam. In my case, I was lucky, and everything worked the way I had anticipated. I then used this same latex covered sample piece to trial my painting methods. This is where I learnt something. Certain colours didn't work the way I wanted, so I was able to find a colour scheme that looked the way I wanted it to through the trial and error process.