In simple terms, there are two ways to do it.
1) Double/Simultaneous close.
You got property under contract from seller and find a buyer to flip to.
Your buyer comes to the close (or deposits his money into escrow) at 10:00AM.
You meet your sellers at 1:30PM and use that money to pay them.
Once they leave, your attorney cuts you a check for the difference, less fees.
Your buyer gets the deed.
You buyer never knows what you made.
Your seller doesn't know you just resold.
In a double or simultaneous closing, the closing agent will take the proceeds from your tenant/buyer and pay off the seller.
No money directly from you will be necessary; therefore credit will not be a problem.
Most experienced closing agents will be too happy to do a double closing, as they'll be picking up 2 sets of fees: one set between you and your seller, and one between you and your tenant/buyer.
The best thing you can do is to select a closing agent (in my area we use title/escrow companies) who can accommodate the transaction.
Talk to them in advance and develop a relationship.
Then use them over and over.
A good closing agent is invaluable! 2) Assignment.
You got house under contract from seller and find a buyer to flip to.
You call up your investor partner and tell him you've got a deal to flip to him.
Your contract already allows you to assign this deal to someone else, so no sweat.
He takes your place in the deal and closes with seller.
He then pays you your "fee" (whatever you two have agreed on).
If you're a total beginner, I would suggest you flip a deal via an assignment instead of a simultaneous close.
It's a little easier.