Celsius and kelvin
A kelvin is exactly the same size as a degree Celsius, but the kelvin scale begins at absolute zero, the coldest possible temperature. Converting between the two scales therefore means adding or subtracting 273, never multiplying.
A worked example
To change 25 °C to kelvin, use K = °C + 273, which gives 25 + 273 = 298 K. To change 350 K back to Celsius, use °C = K − 273, which gives 350 − 273 = 77 °C. A temperature increase of 10 °C is also an increase of 10 K, because the individual degrees are identical in size. Recap: add 273 to convert to kelvin, and subtract 273 to convert back.
The mistake people actually make
Gas-law calculations always require kelvin. Warming a gas from 20 °C to 40 °C does not double its absolute temperature, which only rises from 293 K to 313 K. Some data booklets use 273.15 instead, so use whichever value the question provides.