George Polya's 4-step problem solving framework is the classic. Understand the problem, make a plan, carry out the plan, look back. Below are student-friendly prompts for each step plus a worked example. Print as a poster, hand-out, or anchor chart.
1Understand
Read the problem. Then read it again. What's actually being asked?
Ask yourself
What is the question?
What do I know?
What words confuse me? (Look them up or ask.)
Can I say the problem in my own words?
2Plan
Pick a strategy. There are many, and more than one might work.
Strategies to try
Draw a picture or diagram
Make a chart or table
Guess and check
Work backwards from the answer
Find a pattern
Solve a simpler version
Use an equation
3Do It
Carry out your plan carefully. Show every step. Don't rush.
While you solve
Write neat, line by line.
Label your numbers (units, names).
If you get stuck, go back to step 2 and pick a different strategy.
Mistakes are clues, not catastrophes.
4Look Back
Check your answer. Does it actually make sense?
Verify
Did I answer the question that was asked?
Is the answer reasonable? (Estimate it.)
Can I check by working backwards?
Is there a different strategy that gives the same answer?