Hello,
Go to Posts > edit a post > select Excerpt in Screen Options at the top:
http://storage4.static.itmages.com/i/15/0609/h_1433857717_5109604_7816da6497.png
and add an excerpt of the post under the text editor:
http://storage4.static.itmages.com/i/15/0609/h_1433857824_3639322_ee01b11dab.png
Regards,
Eva Kemp.