Sit down with a patient partner and just start experimenting. Use the basic techniques that you know as bases to build more intricate designs. Recognize the patterns that will help create points for you to build your ties. For example, if you know how to tie a basic chest harness, you know that there's a point in the back and in the front where you can loop through and anchor your rope. Next time you follow the steps to a tutorial, instead of wrapping the rope the instructed way, try wrapping it around in a different way or a different part of the body, then go back to that point in the front or the back. Use a lot of hitches to create smaller patterns, always using the underlying harness to tie on.
Tying is an art form, so you're going to have to get comfortable being creative and picturing designs in your head. Much like drawing or painting, you can use a tutorial to learn the basics to draw something simple, but you'll have to develop your artistic skills to give that drawing dimensionality and complexity.
Instead of following a tutorial every time you want to create something intricate, find an example of something intricate that you like and look at it like an artist. Break it down. Recognize the various knots and hitches that you see, trace the lines of rope and reverse engineer the design knowing what you know about the fundamentals. If you practice by recreating other works, you'll start to get comfortable with wrapping rope in pleasing ways and knowing where to hitch to get the designs you want.
Here's a quick example for how you should look at rope designs:
I've numbered the ropes in the order which they were tied. Knowing that there's a point in the back that I'm looping the ropes through, you should be able to start tracing the ropes to figure out how it was tied.