Seeking Guidance on Client-First Principles and Best Practices

Hello all, new here and just wanted to say hello. I was looking to see if I could find some direction on material for understanding client-first, I love using Relume, but I guess I'm struggling with the sheer volume of classes it creates and want to learn best practices as far as making adjustments to sizes and such where should I be applying these to, etc. Sorry Im super new to this and really just looking to learn the fundamentals. There is a ton of info out there and was hoping to find some guidance from the group. I'm not opposed to a paid course, I'm focused more on value and quality. Thanks!