Many people use the method you mentioned, to make a resignation certificate.
However, if you sign an agreement with your original employer, indicate how long it will take you to leave your job. Within the period, you are not allowed to engage in related occupations, or you have an agreement with your unit, there are training fees to go abroad, or agreements such as patents. If you make a separation agreement and official seal in private, you need to bear legal responsibility and consequences for your future actions.