It is very simple fellow. There are 3 steps. Step 1 make an equation for the statement of your question. Step 2 revise the statement for the value of 1. Step 3 Multiply the equation created in step 2 by the desired value. Let me give an example:
Question: If 20 bananas cost $3 then how much would 50 bananas cost. Step 1 Make a statement
20 bananas cost = $3
Step 2 Make a statement for the value of 1 for the variable on the left side i.e. bananas in this case. That can be done by dividing both sides by 20.
20 bananas/20 = $3/20
1 banana costs = $3/20
Step 3 Multiply the equation with the desired value which is 50 as we want the cost for 50 bananas.
1 banana x 50 cost = $3/20 x 50
50 bananas cost = $7.5
This is the basic method for all questions involving unitary method. If you still do not get it send me a message and i'll explain further. It will be better if you send a couple of questions from your textbook. When i'll solve them, you will get to know it better.