There's a very nice description in the (Disney canon) Star Wars novel "The Rebellion Begins". Essentially you just focus on the holocron using your force-sense and then imagine it opening. Ezra pictures a Lothal flower blossoming but we can safely assume any similar mental metaphor would work just as well.
The transparent object might be worthless, but it was also fascinating to behold. It weighed almost nothing, and each of its many sides was perfectly smooth, without cracks or creases.
Ezra sensed something was inside.
He pressed, pushed, and pried at the sides. That got him nowhere. He would need a blaster or a drill to pierce through that outer shell, and even with those tools, he’d probably destroy whatever was in there.
He tossed the stupid thing across the room. It bounced off a wall and rolled into a corner. Maybe one of the stormtroopers would trip on it. Then it would be of some use.
Ezra closed his eyes and lowered his head. He wasn’t even angry anymore; he was exhausted. He cleared his mind, just focused on his breath. That always helped him relax. Perhaps sleep could take him out of this nightmare. He could wake up and find himself lying in the patch of grass around his tower, where green daisies grew.
The green daisies of Lothal were dazzling flowers. The presence of other forms of life made them bloom. The follicles on their stems were so sensitive that they could detect the breath or heat of a nearby organism. They would blossom for anyone who spent time to observe them.
He imagined peering at a daisy in the grass, watching it open its petals, slowly, like a child flexing its fingers for the first time. The spread of petals revealed a radiant center that shone emerald-like in the morning sun. The sight of this tiny wonder always revitalized him, no matter how hard his day had been.
“This is Master Obi-Wan Kenobi...” said a stoic voice.
As marvelous as the daisies were, they didn’t have the ability to speak. Ezra lifted his head and opened his eyes.
The polygonal object lay open on the ground, as if its sides had been petals of a flower. From its center projected a miniature ghost of a bearded man in robes. His was the stoic voice.