Sass mixins are most useful when they remove repetition without hiding the intent of your CSS. The goal is not to turn every declaration into an abstraction. The goal is to keep common patterns consistent and easy to reuse.
For a small project, start by copying the mixin file into your Sass directory and importing it near the top of your main stylesheet.
@import "mixins";
Once imported, you can call mixins wherever the repeated pattern appears.
.card {
@include border-radius(0.75rem);
}
A good mixin should make the stylesheet easier to scan. If a mixin makes the code harder to understand, keep the plain CSS instead.