You may file a responsive motion and include your counter points in numbered paragraphs which correspond to the numbered paragraphs in the motion your ex filed. This gives you the opportunity to refute or explain each allegation in a concise format which will be easy for the judge to read. He or she will then have a summary of each of your positions prior to the hearing. Make sure to include the case caption and file number on the top of the motion. (party names, county, state, court division and file number).
If your ex is trying to modify custody and you are seeking to keep custody as is you do not need to file a counter motion, just a responsive motion denying his allegations and denying that any change has taken place which affects the children