Ingame mapping is the easiest way to get started, as you already know the environment from playing the game. Of course you are limited compared to more powerful tools, but with serverside build scripts you can still manage to get some suitable results. A mixture of tool usage and ingame editing is what many mappers go for.
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.