Finger picking notation

I'm just learning how to write music and I'm trying to notate finger picking for a guitar. I'm trying to notate a B note as a quarter note on the B string of a guitar and a G note as a 3/4 note on the low E string of a guitar at the same time. When I select the G note and hold (shift + w) to add the step, it also changes the B notes duration. Is there a way to just change the duration of one note being played at the same time as another note? I hope I'm describing it correctly. Any help would be appreciated.