Count the number of students that will be graded. You can make a vertical curve for each class. However, combining all classes under one vertical curve would produce a fairer and more accurate curve. As an example, consider grading a total of 50 students.
Decide the distribution of the grades. Although you can decide to give out as many As, Bs, Cs, Ds, and Fs as you'd like, typically, a grade of 75 percent C is supposed to represent "average," and so, as an example, when grading 50 students, 25 should be awarded a grade higher than 75 percent and 25 should be given a grade lower than 75 percent. You might give five As, five Fs, 10 Bs, 10 Ds, 10 Cs above 75 percent and 10 Cs below 75 percent.
Rank all the students. A multiple choice test gives will allow for an obvious numeric comparison, as in one particular student got the most questions correct, while this other student missed the most questions. Essays can require a more intuitive approach to ranking, as in one student's essay is better than another student's essay, which is better than another student's essay -- and so on.
Award students a grade based on their rank. From the example, the top five students get an A, the next 10 get Bs, the next 20 get Cs, the next 10 get Ds, and the bottom five get an F.
Hand the students their grades. Including the individual student's rank along with the grade will help him accept the grade.