What do I need to know to successfully put different patterns together in a lounge?

[Denyite]

I'm thinking of curtains and cushions and throws. Maybe rugs and sofas also.

It is a bit of an art rather than a science.
You are best to choose a colour palette of 3 colours and work within that.
Then it is a case of balance. With curtains I like to have around 5 pattern repeats so measure the length, divide by 5 and choose fabric.
With other patterns change the scale but keep colours consistent maybe using bold with subtle with a mid sized pattern on a chair or cushions
It helps to be able to visualise things before you take the plunge.
Think also about balance in the room.

17to35 I really appreciate your response but I'm not sure I'm any wiser. It sounds very complex and somewhat beyond me.

Sophie Robinson is Queen of Patterns in the interior design world. She recommends using a Hero Print (for a significant item in your room, curtains or sofa for example) and selecting a tight pallet of colours taken from that print for other surfaces in the room.

A couple of years ago she had a programme Dream Home Makeovers - 6 episodes which are available for streaming on 5.
Might be worth a look to get some inspiration.

Tight colour pallet, as PP said up to 3 colours but actually it's more because you also can have neutrals and natural texture like wood.
Don't mix florals of similar size, you might get away with florals of very different sizes but it's hard. Best to have one floral and then geometric patterns (stripes, checkerboards, abstracts) and textured things in plain colours. B&W geometrics go with most things.

Have you got any patterns that you like? It might be easier with examples