Begin by creating a four staff system in which you label each staff with the four main voice types. Soprano, alto, tenor and bass.
Copy the original solo part to the highest voice part in the Acapella score. This will usually be the soprano voice.
Copy the top line of the piano to the alto part. If the piano part is extremely high or low, transpose it so that it is slightly lower than the soprano part.
Copy the bass line of the piano to the bass part. The bass part should be transferred at the same octave that the piano part is already playing.
Copy the tenor part over while being careful to ensure that all of the chords from the piano part are spelled out fully between the four staves. For instance, if there is a C major chord, you should have a C, E and G somewhere between the soprano, alto, tenor and bass. If you are missing any note, ignore what is written in the piano and place it in the tenor part.