multiply by 100, therefore in the case of 0.125 the equivalent percentage would be 12.5%.
I think of the % symbol as being a constant of .01 or 1/100. This way 100% = 100 x .01 = 1 and .125 = 12.5 x .01 = 12.5%. I don't know if this is a helpful way to learn it the first time but it makes a lot of sense after learning implied multiplication.