You can use the record demo feature to easily record your gameplay to later playback.
If you suspect someone is cheating, or have a question regarding a map or glitch, you can use this feature for easy reference.
Once you record the demo, you can find the files under your hl2mp folder found at C:\Program Files\Valve\Steam\SteamApps\username\half-life 2 deathmatch\hl2mp
Follow the video tutorial below, made by blood. He goes over exactly what you need to know to quickly get started with demo recording.

Click here to submit a demo for a suspected cheater.
Be prepared to submit details such as player name and STEAM_ID, which you can find on players' HLstatsX userpage.

Click here to submit a demo for other requests.
Please prepare a Readme.txt file with your details and question and submit everything into a bundled zip to Submit Page.