Remember to follow the Guiding Principle to record as much information to help you remember why you attached a particular record to the family tree.
Here's an example of a source that can be found on FamilySearch.org. It is related to Fidelia E. Cartzdafner (FS - LCV9-VXN)'s daughter's birth. I had researched Fidelia because she is the daughter of one of my great-grandfathers.
If you'll notice there is no image available for this particular reference, but I'm familiar with this record set that has the System Origin OHIO-VR. As I understand it, these records were extracted before the indexing program began. Thus, this entry is from that indexing set but not directly connected to an image. The actual image has been digitized and indexed in a separate collection.
What is more interesting is there are two records of the birth of a daughter on June 1881 in Madison County to Fidelia and her husband Charles. The first instance was recorded when they hadn't picked out their daughter's name. The second entry is to record the unnamed daughter's name. Some would incorrectly assume that there are two daughters born on the same day, but given the way the records appear, the daughter born in June is one and the same. No twins. Not only do the original images have an entry in the FamilySearch record system, the indexes to these records are also in the system (that's a minimum of 4 'sources' for the same daughter's birth).
Now, a debate could be argued about whether all of these should be attached to Esta and her parents. I'm not going to get involved in that debate. I'm only going to share what I wrote as reason statement.
So how do I write a reason statement for this record?
Index to birth record for Esta Heffley, daughter of Charles and Delia (Cartzdafner) Heffley, born in June 1881 in Madison, Ohio. Index mentions birth date and place and parent's names.
Index is one of a series of sources based on Esta being recorded in Madison County records twice. The first entry was because she was born without a name. The second entry was the recording of her selected name was with a name. She's not a twin, She's the only daughter born to these parents on this date.
Now, is this the most solid proof statement around? Probably not. But, can I remember why I attached a record to Esta and her mother Fidelia? Absolutely. Will someone else understand why I attached the source. Certainly.
And that my friends is how this reason statement fits my guiding principle. Stay tuned for more examples.
If you have something that I'm lacking on this statement, feel free to share your thoughts in the comments below. If you have a "How Do I Write..." example, feel free to contact me through my email in the side bar or in the comments below. Let's help you know how to write your next reason statement.