I have a document that I created called "Return of Possession".  

I just closed on a home in Utah and after recording I went to take possession and the seller was not nearly moved out (I knew that was going to happen because of the final walkthrough).

I'd clarify annual rent is just Base Rent, and may have a 50% signing, 50% at possession depending how long the build out costs/takes.

Your tacit agreement to this is confirmed by your cashing of their checks and allowing them to remain in possession of the property without eviction.

Landlord shall use its best efforts to give Tenant possession as nearly as possible at the beginning of the Lease term.

The buyer gets possession of the property and makes payments over an extended period of time to the seller and when the purchase price is paid in full then the seller conveys legal title of the property to the buyer.

Keep in mind that evictions can be costly, and are about getting possession of your property. 

I know I need to take possession but I'm not sure how.  

In the above, they took possession of the property with a $30,000 1st, leaving them way upsidedown.

If they cannot move the home then you get a Writ of Possession for the lot.