ことにする and ことになる: perspectives on decisions
Distinguish making your own choice from presenting a settled outcome, then compare personal routines and established rules.
State a choice you make
毎朝、10分歩くことにしました。 says you decided to walk for ten minutes each morning. Attach ことにする to the dictionary form.
The negative form works too: 今月は服を買わないことにしました。 means you decided not to buy clothes this month. Preserve the negative in 買わない.
Focus on the settled outcome
来月、大阪で働くことになりました。 announces that you will be working in Osaka next month. It foregrounds the arrangement rather than who decided it.
ことになる does not prove the speaker had no say. Use context to identify the decision maker, and ことにする when emphasizing your own choice.
Compare a personal practice with a rule
寝る前に日記を書くことにしています。 presents keeping a diary before bed as a practice you maintain. The focus is on the ongoing commitment.
この教室では靴を脱ぐことになっています。 presents removing shoes as the established rule for this classroom. Compare it with a personal routine.
Quick check: Which ending highlights your own decision to eat out less: ことにしました or ことになりました?
Show the answer and why
外食を減らすことにしました highlights your decision. ことになりました presents a settled outcome or arrangement.