You should say:

• when it happened
• what happened
• how important your cellphone/smartphone was to you then
• and explain how you felt about the experience

Sample Answers: https://ieltsquangtri.com/describe-a-time-you-used-your-cellphone-smartphone-to-do-something-important/

You should say:

• What the decision was
• When you made that decision
• Why it was difficult to make
• And explain how you felt about the good result.

SAmple Answers: https://ieltsquangtri.com/describe-a-difficult-decision-that-you-made-and-had-a-good-result/