Protestant, or non-Christian?
The latter need only understand that Christ is risen from the dead, then the Real Presence can be founded upon that central and crowning truth of the faith later on.
Protestants though are stubborn people. For one thing when they say that communion is not His body and blood, they are merely saying the truth in their churches, where there is no Holy Orders and so there is no Real Presence either. In order for Him to be present in the Eucharist the presider must be ordained.
You can show till you’re blue in the face how there isn’t a hint of symbolism in the Lord saying, “This is My body,” and that He is recorded saying these words Four Times in the Scripture, which is like saying it with three underscores, but the stubborn Protestant will reserve their right to take it as metaphorical anyway.
When you show them that John substitutes the feeding of the 5000 followed up by a discourse on the Real Presence in his sixth chapter, instead of a fourth account of “This is My body,” they will still resist.
In a real way, to get them to believe in the Real Presence is to get them to believe in Catholicism (or at least in Catholicism or Orthodoxy), and they on some level know this, and so you’ve got that working against you as well. They won’t capitulate their whole theology over just one point of doctrine, no matter how good a job you do at demonstrating that it’s scriptural and historical.
But mainly it’s stubbornness. They stubbornly refuse to relinquish their stubbornness. All you can really do is pray for them.