My crown recently came out. I ordered some cement called prime dent. How do I use it?

I am not familiar with the product you mention, I assume it is a temporary cement.  If so, it SHOULD include directions, including how much of each material (size of scoop, number of drops, amount of time mixing so you don't spatulate too long, etc).If it is a permenent cement, I highly caution against using it.  If it isn't completely removed, the remainder can irritate the tissues leading to bone loss and eventual loss of the tooth.  See a dentist ASAP.

It is a bad idea to try to permanently recumbent your own crown. You can't see exactly where you are placing it and you have no way to clean off all the excess cement. Also why did the crown come off? Is there something wrong with the tooth under the crown? Is the fit of the crown a problem? The cost to reverent a crown in a dental office is minimal. Return the cement and visit your dentist.

You need to have a dentist cement the crown back on if it I fact can be recemented.  You risk the chance of further damaging the tooth underneath