1. Presuming you are not a contractor in the trade or business of constructing, renovating or buying & selling real property (flipping) for a living, &
2. Presuming that you did not sell the property to a related party &
3. Presuming that you made a profit on the sale,
then, what you need to do is to complete Form 6252 - Installment Sales and carry the information with respect to the current year forward to Schedule 8949 and/or Schedule D as a Short-term capital gain, presuming your "flip" resulted in you owning the property for less than 1 year.
The form & the Instructions to the Form 6252 & are with the form and may be accessed at IRS.gov or at the following link:
You will also find a link to Publication 537 at that spot which has a lot of information on installment sales, most of which won't apply to your situation as you are dealing with a short term capital gain.
As you may know, the Installment Sale rules, if you are eligible to use them (see presumptions above) will allow you to spread reporting of the short-term capital gain out over the principal collections on the mortgage. OR,
if it is to your advantage, you may choose to report the entire gain in 2015 if you wish. There are certain circumstances where that could work to your advantage:
If you have a carryforward Net Operating Loss that you wish to use.
If you have a capital loss carryforward that you wish to use.
If you expect that your tax rate will be higher in future years, so you will pay on the gain at a higher tax rate.
If you need to report increased income on your tax return for a variety of non-tax financial reasons.
If after you have had a chance to go over the installment sale material, if you have any questions, please don't hesitate to ask. I'll get back to you as soon as I can.