Designing Clothing with Aphantasia

I have aphantasia, which means I don't form mental images. I also have alexithymia, part of which means I can't imagine. So... how does it work for me to design clothing without being able to imagine or form mental images of things I haven't already seen?

Patterns. Patterns are great, patterns are my friend. They teach me new techniques and new ways of putting things together. Most of the time I use the whole pattern, but sometimes the only way to get the garment I want is to heavily modify a pattern or smush features from two different patterns together. I have a sewing mood board where I stick images of fabrics and designs next to each other until it feels right. Then, when it's time to actually make the garment, it's fine if I did the planning and design months ago, because I already put any relevant notes on the mood board or a text document I keep for designs which aren't fully formed yet. 

Because I'm hand-sewing, and even small lines of stitching take several minutes, I have time to make small adjustments even once I've started, if I think of something I'd like better. This comes up most often when I'm deciding whether to use the closure method from the pattern, or to do something else. I like clasps and I dislike making buttonholes, so I am often willing to change the closure method to something that fits the garment better or is more suited to my tastes (or those of the person who will wear the piece).

I've also started designing outfits from the ground up, using reference images (like a character from my favorite TV show). Over the course of months I've looked for fabrics that fit the design I'm trying to achieve, and which I can obtain in the correct quantities (which is tricky for something I'm designing myself without being able to get length info from a pattern packet).

Once I start working, I have a sense of whether things are going how I want them to. I can test out color combinations by just sticking fabrics next to each other and looking at them, and I can test fit by pulling the unfinished piece on at different points and making sure it seems loose enough to fit. I also pay close attention to the guidelines for what fabrics to use. 

I've never been able to imagine or form new mental images, so it's not like I used to be able to design differently and then had to adjust. I just iterate, trying things until I'm sure that this is the one that works, then I go for it.