It's all about your opinion
Mixing fabric patterns is not about 'right' and 'wrong'.
It's about training your eye and forming your own opinions.
When you combine other patterned surfaces with fabric patterns things can become, well, interesting!
Suppose you've inherited a beautiful patterned rug and you want to use it in your living room.
Ten minutes later you are convinced you need to throw out the couch and wing chair, and completely redecorate the room. Your high expectations have turned into a nightmare of conflicting shapes and competing colors.
Here are a few guidelines.
Compliment, not conflict.
When you mix patterns, have several which compliment one another with none fighting to be the dominant pattern. If one stands out and attracts attention to itself, then choose the others so they play minor parts.
Balance is the key word here. Similarly, if you use one large pattern, don't use another large scale one which may compete with it. Go for smaller patterns. Some manufacturers produce designs in both large and small formats.
Or use plain fabrics with your large pattern. This is by far the safest option.
An easy way to combine patterns is to use ones of a similar type, such as ethnic or oriental. Or use patterns with a common theme, such as wild flowers, seascapes, or paisley designs. But again, make sure they don't fight with one another for attention.
Matching different materials
Some companies produce fabrics and matching wallpapers. This gives 'life' to a room which can be very attractive.
It is a technique used a great deal in France, where 'toile' designs (often rustic scenes in two tones) are used to create a unique effect.
Care with small patterns
Be careful with very small patterns. From a distance the design can become indistinct, and the pattern appears more as a texture. Make sure you get a large enough sample so you can see exactly how it will look.
Play it safe
If you are unsure of mixing patterns, then there is no harm in playing safe and just using one pattern in the room. For example, you could use a favorite pattern for your curtains, and then extract three colors from the fabric to use for your carpet, walls and plain or textured upholstery.
Learn about color
If you're not sure how colors work together then take a look at the section on this website about color schemes. You'll find some easy theory, and also some practical color charts. Although it may seem like an effort (we all want easy options when we look at websites!) it will be well worth leaning about color.
Many people are confused by drop repeats, or half drop repeats which is the most common type. You can find an explanation on the half drop page of this website.