How to Mix Patterns in a Room You Already Live In

A striped cushion bought two years ago, a floral one that arrived since, a patterned cloth from somewhere else. They are already in the room. Whether they work together is something you can test with what is there, changing one relationship at a time and looking again.
Begin with the pattern already in the room
Pick one patterned thing you would keep even if you moved house next month. Notice where it sits in your actual line of sight: the cushion you see from the doorway, the mug on the shelf you pass on the way to the kettle, the cloth on the table where you eat. Position sets viewing distance, which changes how much of the pattern you take in at once.
Leave the rest of the room alone for now. Buying a set of matching new things is one way to settle a corner, and it is worth trying the pieces you own first, since anything new has to answer to the room anyway.
Find a colour connection without matching everything
Put the two patterns side by side and look for one colour they can share, not all of them. A wide blue stripe and a floral with rust petals and blue stems already connect through the blue. One blue can be deeper and greyer than the other and the pair can still read as related, so try related shades and see whether you like the connection.
A design studio's notes on working with pattern treat colour as one of several considerations alongside scale, application and balance. That framing is more useful than a fixed palette: you are adjusting a relationship, and the adjustment can go either way. A neutral that appears in both patterns is itself a shared colour and can carry the link. If the two have nothing in common at all, that is worth knowing before you build a corner around them.

Compare the size of the marks
Stripes, gingham, spots and florals are loose categories. Two florals can differ enormously in the size of each bloom, and a broad stripe beside a narrow one behaves differently from two stripes of similar width. Textile ranges often list small prints, stripes, geometrics and textured plains as separate groups, which gives you a vocabulary for comparing what you already own.
Put the pieces together and step back to your usual viewing distance. Pairing a large mark with a small one is an option that often reads clearly, and two patterns of similar scale can also work when their colours sit close and there is space between them. Try both and keep the one that looks settled from where you sit.
Give the eye somewhere quieter to rest
A plain surface between two patterned objects is one way to keep each of them legible. The interval does not have to be bought: a bare stretch of table, a plain wall, the undecorated end of a sofa and a length of plain curtain all do the job.
Leaving part of a surface plain is a choice, not a requirement. A layered, busy corner can be the point, and plenty of rooms carry three patterns at once. What the plain gap buys you is comparison: with a clear area between them, you can see whether the two patterns are doing what you want. Clear a table completely for a day and see what changes; put things back if the answer is nothing.
Try one pairing in an ordinary corner
Take a corner you use and change one relationship at a time. Suppose a pale sofa has a wide blue stripe cushion at one end. Bring in a smaller cushion with a rust and blue floral and put it at the other end, leaving the middle of the sofa plain. Sit where you normally sit. Then move the floral cushion closer to the stripe, keeping everything else identical, and sit down again. Compare the two positions and decide which spacing you prefer.
A second test changes colour instead of distance. Swap in a floral with a deeper blue and see whether the link gets stronger or the corner goes flat. Change one thing at a time, or you will not know which change did the work.

Look again when you actually use the room
Light moves through the day, and a pattern that reads crisply at eleven in the morning can soften into a single tone by evening. Use the room as you normally would for a few days: sit down, walk past, put a cup on the table, pick up the book you are halfway through. Look for anything you want to change when you see the room from a chair or walk past it, as well as when you stand in front of the arrangement.
A phone photograph helps you compare two arrangements if you take it from the same spot in the same light. Its framing and processing can differ from what your eye sees, and colour shifts on a screen, so compare at the distance you view the corner from. If you like a fuller, busier room, that is a legitimate preference; the aim is a room you can live in, and an empty surface is only one way to get there.
Keep the pairing, change one thing, or stop
Three endings are reasonable. If the corner looks settled from where you sit, keep it and stop adjusting. If something still nags, change one element: move the smaller object, take one pattern out, or open a plain gap between them. If two patterns will not settle, put the newer one in another room and leave this corner as it was.
Adding a third pattern is also an option, if that is the room you want. This method does not require buying one. The next step is small: put the two cushions together tonight, sit where you usually sit, and move one of them once. For a closer look at how texture and construction hold up at that distance, the details that make handmade feel different covers what changes between near and far. If the wider question is light and atmosphere more than pattern, the atmosphere of home takes that on.