I think “properly disposed” means understanding oneself to not be in a state of mortal sin, as would apply to a Catholic going to Communion.
One commentary about “demostration of the Catholic faith” has:
“… This does not mean that the person needs detailed knowledge about the theology of the sacrament in question, but only that the recipient believes in the essential meaning of the sacraments. For example, in reference to the reception of the Eucharist, it suffices that the recipients believe the consecrated bread and wine is spiritual food, the body and blood of the Lord. (See cans. 889, §1 and 913, §2). …”
(John Huels, The Pastoral Companion, Franciscan Press, 1995, ISBN 0-8199-0968-8, page 347.)
The canons he refers to, 889 and 913, could be looked up in the Code of Canon Law at
vatican.va/archive/ENG1104/_INDEX.HTM .