How can I proceed with criminal mischief charges against my ex?

You need to report this to the police.

Call the cops and make a complaint. He should get arrested.

You have three options: 1) you can file a criminal complaint against your ex-boyfriend with the police; 2) you can file a family offense petition in Family Court; or 3) you can simultaneously exercise option 1 and option 2. The first option could result in felony charges (based on the value of the damaged property) and/or misdemeanor charges filed against your ex-boyfriend in Criminal Court. He would face the possibility of a criminal record, jail time, probation, restitution, and all other sentencing options available to Criminal Courts. If charges are filed, the Criminal Court most likely would issue a temporary order of protection while the criminal case is pending, and a final order of protection when the case is over, directing your ex to stay away from you and to avoid communicating with you in any way. The second option would require your boyfriend to appear in Family Court. The Family Court most likely would issue a temporary order of protection while the family offense proceeding is pending, and a final order of protection when the case is over, directing your ex to stay away from you and to avoid communicating with you in any way. If you proceed in both Criminal Court and Family Court, the cases will most likely be joined in an Integrated Domestic Violence ("IDV") Court. One judge would preside over both proceedings (one of which would be prosecuted by the District Attorney, the other of which would be prosecuted by you or your lawyer). Where you proceed could depend on how much you want to jam up your ex-boyfriend and other factors. Ideally, you should consult an attorney who is familiar with handling domestic violence cases in both Criminal Court and Family Court, to help you determine which venue or venues is consistent with your goals.