It sounds like you want to make a trigger that declares victory for a certain player. To do this, you need to go to the "Triggers" section in the map editor. Here you can set lots of triggers which are mainly used for scenarios. Each trigger has one or more conditions and one or more effects. For the effects to actually happen, all of the conditions must be met. If you want to make custom victory conditions, you need to make what you want to happen in individual "conditions". You then need to make an effect labelled "Declare Victory". You then set the player to who you want to win, when the conditions are met.
Example: You want player 1 to win the game when they kill a certain unit. You need to do the following steps:
- Open the Scenario Editor from the Main Menu.
- Go to the triggers menu. Click "New" under the triggers box. (Note: you can change properties of the trigger here, such as the name of the trigger and whether it loops or not.)
- Click "New Cond" to make a new condition. Set the condition to "Destroy Object". Click "Select Object" and then select the object you want to need to be killed. (Note: by clicking "View Object", you can check if you have selected the right guy.
- Click "New Effect" to make a new effect. Set it to "Declare Victory". Set the player to "Player 1".
You're done! Of course, if you want the victory conditions to be different, just change what the condition/s are set to. Most of them are pretty self explanatory, and easy to use. Almost everything I know about the editor I found out myself. It's not too hard to use, but browsing some wikis may help.