A "sandbox" allows your user (or yourself!) to play around without damaging important things. To this end, Reva makes it pretty simple to seal off all words but a limited set you wish to expose.
The first step is to create a context which will contain all the words you wish to expose:
"~public" was chosen by way of example - any name will work.
After you have the context, put the words you want to expose in it, either by creating them there or by moving words from other contexts into the one you want, or by creating aliases:
~public | make this context the active one | create new words: : foo ... ; : bar ... ; | move other words: ~ to~ ~public words exit~ | make an alias: alias: bye! bye
Finally, make the context you have chosen the only one available: