First off, Precure has a strong tendency towards boots over other types of footwear, and fingerless gloves or arm covers (esp. with a heart or other symbol on the back of the hand) over traditional gloves. If you DO have Cures with gloves with fingers, there's a very strong trend towards short white gloves with a ruffled trim. Boots also tend to have separately colored toe tips, often with a little shine on them. Also, a set of knee high, thigh high, and short w/socks boots for a team.

There's also a really strong trend towards choker necklaces (with or without bow on back), as well as earrings and a hair dec in the shape of a heart OR the Cure's personal symbol. Part of a larger trend of assigning each girl a unique little symbol/emblem in addition to theme color. (This can show up in attacks and especially in merch!). The transformation item is also always part of the outfit; most of the time it's in a pouch at their waist, but it can also be on their chest like a brooch, Sailor Moon-style (but even then there might be pouch anyway). (Obviously there are a few outliers, like Healin Good or Hirogaru Sky).

Some series have much more complicated and/or cartoony hair than others, but overall it's pretty stylized. Hair is almost always a much brighter color than their civilian hair (except for Futari wa), and most often their theme color (or blond...). Some hairstyles tend to repeat (e.g., very long hair down, big high ponytail, pigtails). Also a general trend towards complicated bangs + sidebangs. More recent seasons also tend to have died streaks and/or gradients in different colors.

Precure have a very strong tendency to wear skirts and not shorts. It's also pretty unusual for Cures to have a long skirt instead of a shorter one or a high-low one. Long skirts usually suggest maturity, so it's basically just Big Sis / Mentor Cures (...and Sango and Milk. Strong purple bias for some reason). Skirts are almost always layered, esp. with ruffled trim/underskirt. Precure also has a strong tendency towards skintight shorts underneath skirts, although not as much in recent years. No pants (except Black Pepper, who is in the category of non-Cure ally), but sometimes tights/leggings.

It's also extremely uncommon for Precure to have long sleeves! Precure tends towards puff sleeves, spiky/wing shaped sleeves, ruffles, or just open shoulders. Overall, Precure tends to have lots of ruffles, esp. ones stylized into a feather shape, as well as hearts. Bows or other ties on the chest are also very common, but tbh this is kind of a larger magical girl trend...

As an overall tip for creating Precure designs from existing characters, I tend to start by asking "how would Precure render their canon hair and design?" as a base. For example, Mami's spiral hair is heavily inspired by Macherie's (rather than Lemonade's or YumYum's, other options I considered), and Zakuro and Kyoko's ponytails are based on Bloom, Aqua, March, etc.

I try to keep the silhouette similar to the original design and keep any iconic outfit pieces + symbols from canon (e.g., Zakuro keeps her bare midriff and thigh high boots, Mabel keeps her shooting star symbol + theme colors, Sayaka keeps her cape). I also try to draw from other canon outfits/forms to give more room for unique designs; for example, I drew from the beta + princess designs for the Sailor Senshi and the Trickster Mode designs for the Homestuck characters.

Speaking of motifs, it's also helpful to settle on some kind of theming for the "season" as a whole and for each individual Cure to stop them from looking too generic (e.g., food types for Delicious Party, objects in space for Star Twinkle, sailor/sea/island related items for Tropical-Rouge). Some series go very hard into this theming, and others have more generic designs, but even then there are repeated outfit elements that make the team look cohesive (e.g., the wings + tiaras in Smile, the side flowers in Heartcatch, striped ribbons in Wonderful). Some seasons have very uniform outfits (e.g., Smile, Happiness Charge, Yes 5) and others are very distinct (e.g., Star Twinkle, Tropical Rouge, Go Princess).

For my designs, the Mew Cures have small elements to suggest their theme animal, and the designs as a whole have repeated elements among them (e.g., bodice with horizontal seams, off shoulder puff sleeves, and the little X motif). The Madoka Cures also have the bodice piece, but with translucent fabric and some form of crinoline (...this is not an element which would actually animate will imo, but hey!!! It's not a real series!!! It's inspired by the look of a Soul Gem/Grief Seed). The HS Cures are strongly themed around sweet + Halloween costume, and they all have striped tights, pointy shoes, and the candy bow.

All of these are my tips for creating designs that look Like Precure TM. BUT I do want to note that you shouldn't feel compelled to stick super closely to canon!!! Precure has basically zero diversity in body type, and the only Cures with a darker skin tone are Soleil and the 2 single-episode Hawaiian Cures from Happiness Charge (notably NOT Cure Nile or the team from India......).

Also, I tend to like the girly aesthetic of Precure and appreciate how even the guys can have more feminine design elements without it being "embarassing" or shameful (esp. Infini, but also Wing, Connect, and Black Pepper a little), but it does kind of... not really leave room for gender non-conformity in the other direction? I think we should let more girl Cures be butch AND let guy Cure wear skirts without it being a joke... And just enby Cures in general, outside of the purely-subtext Infini???