Well, to do this takes a little practice, and it is very helpful to have some pedal technique. When you're playing with both hands on the same manual, you can take the alto and tenor in whichever hand is most convenient. But when you're soloing the melody on another manual, it is often difficult to reach the three lower voices with the left hand alone. So you need to practice playing the bass line on the pedal, the inner voices with the left hand, and the melody with the right hand on another manual. Sometimes it is still difficult to reach the alto and tenor with the same hand and then you need to change them a little bit to make sure you can get all the notes somehow, because it will not sound so good if, for instance, you leave the third out of a chord.
Also, keep in mind that there are lots of ways to vary the verses on hymns. You can take both hands to another manual, change registration, change the harmonization, sometimes play just the melody on the pedals alone on full organ. . .It's best to study what the words are saying in each verse and then try to make the organ somehow reflect what the words are saying.