How do you do percents? If you'd like to add percents by simply adding them together.
For example: 25%+50% = 75%.
The same is true for subtracting percents.
Now, if you'd like to find percent of a value, then that usually applies different rules.
Say, for example you'd like to find 25% of 37.
You will first convert 25% to a decimal value. Sine 25% means 25 out of 100, 25% = 0.25
Then, you would multiply (0.25) and 37 which would yield 9.25.
Therefore, 25% of 37 = 9.25.
In most instances, you must change your percent to a decimal in order to perform operations with that percentage.
First of all you need you need to know two figures. One should be the total amount out of which you are finding the percentage and the mere amount the percentage of which you have to find.

2
If the total amount is other than hundred then you have to first put this in fraction form. This is done by placing the total or the larger amount in the denominator and the smaller amount or the one for which you have to calculate the percentage in the numerator.

3
Once you have put the two amounts in the fraction form, make the calculation manually or by using the calculator if the working is a complex one

4
As you will calculate, the figure you will get will ofcourse be in decimals. Now you will multiply the figure with 100 and you will get the percentage. For example:
135/150 = 0.9
0.9 x 100 = 90% 
5
Now if you are to find the amount against a certain given percentage such as 17% of 150 then you have to simple have to multiply 17% with the total amount in the following way:
17% x 150
17/100 x 150 = 25.5 
6
In the above step, make sure that when you are finding the amount instead of percentage, like you have to find your marks in figures from the percentage, simply divide the percentage with 100 and multiply it with the total amount.
If you mean doing percent's from fractions I will be glad to show you!
So for example take the two fractions
4/5 or 7/10 All fractions have to ability turn in a decimal! So now put these in a calculator and you should get
4/5= 0.80
7/10= 0.70
Now in order to turn both decimals in percent's we are going to multiply each decimal by 100 and add the percent sign and you have your percent!
0.80x 100 = 80%
0.70x 100 = 70%
To determine percentage of a number, you must multiply the number by the decimal form of that percentage.
First, find the decimal form of a percentage:
Ex 1: 15% = 0.15
Ex 2: 7% = 0.07
Basically, move the decimal two places to the right in order to create a decimal form of percent (or multiply your percent by 0.01)
Then multiply the number whose percent you want to find by the decimal form of the desired percentage.
Ex 1: 0.15 x 200 = 30 (15% of 200 is 30)
Ex. 2 0.07 x 50 = 3.5 (7% of 50 is 3.5)