This video introduces some of the more frequently used build commands and shows what they do.
Things to consider:
Save your map ingame by pressing 8 (in OpenSpades) or F1 (in Ace of Spades Classic).
The video doesn't explain the /god command, as it is applied automatically (in the build gamemode that is used for Custom Games). If you run your own server instead, don't forget to use /god for unlimited blocks and to include all necessary build scripts. Godmode is a bit bugged in the current version of OpenSpades (see github issue #254), some other build commands too.