In order to manually control your blog post's excerpt there are 3 steps:
1.) Inside the WordPress Post editor page, make sure you have activated the Excerpt box by clicking on SCREEN OPTIONS in the top right, and then checking the box for "Excerpt" as shown in the screenshot below.
2.) Checking this option will cause the Excerpt box to appear below the Post editor, as shown below.
3.) Now you may type in whatever text you would like to have for your blog post's excerpt. This can be custom text, or any text you would like to appear on your blog's front page.
After you UPDATE your post, this is how it will appear on your blog's front page:
If you are using a "Featured Image", that featured image will also show on the your blog's front page.