At least in C172 POH there is special section under Emergeny Procedures called Rough engine operation or loss of power - perhaps it's also in C150/2.
But basically, BackPacker covered the entire procedure. Just to remind you, if you will apply carb heat, the RPM will drop initially, even if carb is full of ice. The proper procedure is to apply carb heat and then wait 10-20 seconds and monitor for any rise in RPM which would be a sign of carb ice.
Also, descend from high-altitudes with mixture fully rich may cause engine roughness - most instructors teach fully rich at TOD, but that actually overcools the engine and the combustion isn't as clean as it would be with a leaner mixture. The way I was taught is to leave the mixture at TOD where it was during cruise, and moving the lever 0.5-1 cm to the rich every 1000 ft, untill reaching 2500-3000 ft, where mixture is set fully rich.
As BackPacker also said, you could try different throttle/mixture settings, which is actually recommended procedure by Cessna, if I remember correctly. And when/if you find the setting that gives you the least roughness, at that setting you fly to the nearest airfield.