Here's an example for you:
mat = [[10, 20, 30, 40], [50, 60, 70, 80], [90, 100, 110, 120]]
flatlist = 
for sublist in mat:
for item in sublist:
Suppose you have a 2D list called mat, all you've to do is, crawl through the list elements and flatten the list by appending the list items one after the other.