First: what I am about to say is a generalization and there are exceptions. but generally...
Generally, mountains affect climate. Clouds and rain are carried by winds that tend to blow in the same patterns. When clouds hit mountains, they dump their rain, causing lush forests. But on the other half of the ridge, there is no rain and a desert is created. a wonderful example of this is Northern California vs Nevada. As you go over those mountains you go from forests, strawberry farms, and wineries to slat flats and cacti.
Certain general worldwide patters (caused by the earths rotation) are shown on this map:
Attachment 42596
Exceptions are common, for example this is how winds generally are:
Attachment 42597
Now, because your world is flat, your winds can follow any pattern you want, or they can just blow randomly, but if they blow randomly, then the entire planet should have more or less the same rainfall. My suggestion would be to make a wind direction chart and mountain ranges that fit with the climates you already have.
These rainfall areas will affect multitudes of other things as well. Winds coming off deserts will be warm; one of the reasons for the warmth of europe is the wind from the sahara.
Here, for reference, is a map of Europe's Topography:
And here, a map of the Rainfall, with a strong correlation between changes in altitude and a massive dumping of rain.
Once again, there are exceptions, but just think about this before placing major mountain ranges.
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