Adding Notes

I'm a beginner to musescore and I'm having trouble adding notes to my score. I'll click on a blank measure and the whole rest/measure will highlight blue, then I'll turn on my notation and select a quarter note, for example, and rather than adding a quarter and changing to a quarter and half rest in the measure, it just keeps adding time to the measure (as in a quarter note followed by a quarter rest, half rest, and another quarter rest). When I look up video tutorials they seem to not have any issue with this. How can I go about fixing this?