Assuming you already have a navigation bar in your blog’s theme. Go to the CSS of your theme.
If using Blogger, go to edit template and find the CSS of your navigation bar.
Use inspect element tool in Chrome and Firefox to correctly identify your navigation bar’s CSS.
Now simple add
How it works?
It in the position part. When we set the position at FIXED the browser will keep the navigation bar’s position at one place even when the page is scrolled through.