Yes, it is true, you must first create it in your mind - or there is virtually no chance that you'll get it.
And it should be an "expansive" vision, one that is most likely possible, where you're not limiting yourself to a life of mediocrity and regrets for later looking back and seeing there was much more to be lived.
Examples and how to create a vision follow.
WHAT IT IS
From Jack Canfield's book The Success Principles, one of the top recommended books:
"The theme of this book is how to get from where you are to where you want to be. To accomplish this, you have to know two things - where you are and where you want to get to. Your vision is a detailed description of where you want to get to."
Once you determine this, then you simply write down what it will take to get you there, using the formats in the Planning section.
HOW "BIG" SHOULD IT BE?
"If you limit your choices only to what seems possible or reasonable, you disconnect yourself from what you truly want, and all that is left is a compromise." Robert Fritz, "The Path Of Least Resistance"
Start, at least in your first efforts, from what you truly want, without throwing in your imagined limitations. You can have some personal visions and you can also have some world visions, but only if you are so inclined - it is all up to what you truly want, not what you think the world expects of you.
NOT BASED ON THE PAST
"Create your future from your future, not your past."
Werner Erhard, EST
My Vision Quest, 8/21/02 - An actual "vision quest" in a different format, but a vivid one. (From the About Me Notebook, though it could fit in the LifePlan Notebook with a copy probably, kept for review, in the Reminders Notebook.)
FOR YOUR RELATIONSHIP
The Grand Vision For The Relationship - An actual one, envisioning a grand relationship. Includes "Our highest intentions and commitments for the relationship," as a way of further clarifying it. (It is not necessary to have such a grand vision, as the first priority is to actually reach an agreement on the vision.)
FOR MY CHILD
A Letter To My Dear Child (To Be) - "My wish for you is that you will...", so starts the vision for your child's life, in a letter that can be so meaningful to a child and which will guide that child to live a life more like that. All children would like to have something like this from their parents.