I'm not an amazing vocalist and definitely not an expert in Solfa, but my basic introduction from the Sound of Music and attending my son's Let's Play Music Classes have opened my eyes to its beneficial application in piano lessons, and I've been incorporating it more and more each week as students play their pentascales. This helpful article by Nicola Cantan gives some great tips on how to start using solfa with beginning piano students.
Learning Melodic Dictation Step by Step
After having my students complete the Good Better Best Rhythmic Dictation activity that familiarizes them with the rhythm of this chant, we move on to melodic dictation using solfa. The teacher (or one student in a group setting) plays the "Good Better Best" rhythm on a set of tone bell(s) hidden from view and then the other students echo what they heard while singing and signing the appropriate solfa symbol(s).
Start simple and guide the students by singing the selection of pitches before each example played (Do-Re-Mi, or Do-Re-Mi-Sol, etc.). For example, sing and sign Do-Re-Mi and then play the "Good Better Best" Rhythm on Repeated Notes of either Do? Re? or Mi? and have the students guess which pitch you played by echoing as they sign and sing. As students become adept at identifying the pitches, gradually increase the difficulty by adding additional tone bells for them to sing and sign.
1- Repeats only Do? Re? or Mi?
2- Repeats and/or Skips Do-Mi
3- Repeats and or Steps Do-Re-Mi
4- Repeats, Steps and Skips Do-Re-Mi
Solfa and Staff Notation
I have students sing solfa as they play their pentascales each week which helps them to make a correlation between the notes on the staff to the moveable do solfa pitches, but it is helpful to point out the correlation between the solfa steps and pitches in the C scale as they are used in this song. Once students are comfortable identifying the solfa symbols for the short melody, they can also move small objects on the correct lines or spaces of a paper staff to "notate" the melody they heard. For example in the key of C, Do-Re-Re-Mi would be C-D-D-E on the staff.
Melodic Dictation App for Beginners
Another helpful free tool that I have my students use during piano lab is the "Bubble Tones" app which includes gradually sequenced ear training exercises. It is especially helpful if you sing or hum along as you play to "anchor" your hears to detect the pitch differences. I love how this app begins simply with just differentiating between 2 possible pitches (Do and Re), but later gradually adds an increased range of pitch choices, intervals and number of notes to remember with increasing difficulty as you move up the levels.