First off, you'll want to find and copy the URL (Uniform Resource Locator).
Edit>Copy on the menu bar or Ctrl+C on a PC or Command+C on a Mac (Command key is the swirly square thing on either side of the Spacebar)

Go to the Blogger Create Post page and type your text.

Select the text you'd like to link.

Click on th e"Link" button (circled here in blue).

A box vaguely similar to this one will pop up (the color is just particular to my personal Firefox browser, not important)

Paste teh URL you copied earlier here.
Edit>Paste on the menu bar or Ctrl+V on a PC or Command+V on a Mac (Command key is the swirly square thing on either side of the Spacebar)

Click "OK"

You should now have your link!