Well, if you navigate through the drop-down menus , and then select the guideBGM option (Options > Show BGM Guide), it opens another smaller window with the guideBGM interface, like so.
A brief overview of the options presented here.
>Manual Recording: this is the default option. Press R to record.
>Semi-auto Recording: Pressing P will make the guideBGM play once. It's up to you from there, you can listen to your sample to check for recording errors, or you can move on. (This is if you really want to pay attention to detail)
>Automatic Recording: It plays on a loop, and automatically cycles through your reclist and saves recordings. (This is if you want to record a voicebank quickly, and then go back and fix errors later - I use this method~)
>Invalid selection: Idk. I've never selected it tbh

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Then the little box underneath these thingies lets you select which audio file you want to use as a reference.
Hope this helps~
[EDIT: now the image should work]