Word problems are a great way for students to identify real world applications for the information they are learning in the classroom -- while helping them develop critical thinking skills. To write a word problem, analyze the way you would solve it yourself, and decide on the best method for your students to use.
Identify the math skills you would like students to work on. For example, if you want students to focus on adding and using images to help solve problems, your focus will be on writing a paragraph that accomplishes that goal.
Choose a main character for your word problem. Pick the number of objects and type of object that he has. The best objects are easy visuals for children to identify and draw, if needed. For example, your first sentence can be "George has 6 red balloons."
Choose a secondary character and identify the number of objects he possesses. For example, "Alexa has 7 purple balloons."
Write a question at the end to the word problem. An example of end questions is the question, "What is the total number of balloons?" Use math vocabulary that is age-appropriate for the students.
Add further instructions to help students achieve the result you desire. For the sample problem, you would want to tell students to draw the balloons to help them solve it. Also, encourage students to write the number sentence that goes along with the problem.
Read through your word problem and try it. If you are not clear in your instructions or you decide that the problem is too complicated, revise it to suit your students.
Ask students to write their own word problems as an assignment to turn in. This will allow you to see if they grasp the concept. Use these student-created examples as practice problems in the future.
TL;DR (Too Long; Didn't Read)
Introduce students to the concept of a word problem within the context of what they are learning. Doing a group example will help them better understand the material.