I looked at OpenSTA quite a while ago so havent used for quite a bit.
Basically though OpenSTA goes through the same process as all other Load Test ing software.
Capture a Recording of your user/business process/transaction using 'Script Modeller'
Paramterize variable data items such as user id/password where you require unique data.
Test amended script works by running/playing it back through script modeller
Group Tests together and run them via Test Commander, defineing how many user you require to run each script.
Technically running the 300 user is achieved by you using what OpenSTA term as 'injector' machine which are simply other machines on your network which have the ability to run your scripts (this can be one or many PC's). Each injector can run multiple instances of your script, thus simulating X number of users.