The principles to successfully mixing patterns are color, scale, and shape.
Color - there has to be a cohesive color match and complement.
Scale - mix a variety of scales (size) of pattern.
Shape - mix geometric, abstract, stripes, and plaids with organic, botanical, or florals.
It is relatively simple to mix patterns if you start with a color scheme. Use the colors to select a variety of fabrics with those colors. Begin by choosing any fabrics that appeal to you with the colors with no decision on whether you will use it or not. Look for solids, textures, and patterns.
The examples below show the mixing of patterns for projects. It is important to consider rugs and other elements in the space when deciding where to use the fabrics. It is my opinion to go with a textured solid or subtle pattern on large pieces such as a sofa or sectional. Use the patterned fabrics on chairs, pillows, window treatments, etc.
When I work with clients I browse fabrics on line, and I order many memo samples. Then I go through the samples and funnel down the fabrics that will work well for the application. The only disadvantage to online browsing is not being able to feel the fabrics. I enjoy looking through hundreds of fabrics without going to the Design Center in Boston.
Here is a video I did YEARS ago. I was thinner and without my current gray hair; but the information has not changed.
Have fun with fabric. Don't rush the selection process. If you feel that you need or want help reach out to a designer for assistance.