best practise to apply "fill in blanks forms"

That just sounds like a regular form? And then a page for showing that record from the database? Where is the tricky part?

An HTML form can contain virtually any kind of data – it’s not limited to just input fields and labels.

This sounds like you just need to wrap the ‘story’ (whatever the text that has the blank spaces in it is) in a form element, use text input fields for the blank spaces to be filled out and style those fields as inline elements so that they look like part of the text.

For example (using The North Wind and the Sun as the story and Tailwind for styling):

<form action="dostuff.php" method="post">
	<h1>The North Wind and the Sun</h1>

	<p>The North Wind and the Sun were disputing <input type="text" name="f1" id="f1" class="inline mx-1" />, when a traveler came along wrapped in a warm cloak.</p>
	<p>They agreed that the one who first succeeded in making the traveler <input type="text" name="f2" id="f2" class="inline mx-1" /> should be considered stronger than the other.</p>
	<p>Then the North Wind blew as hard as he could, but the more he blew the more closely did the traveler <input type="text" name="f3" id="f3" class="inline mx-1" />; 	and at last the North Wind gave up the attempt. Then the Sun shined out warmly, and immediately the traveler <input type="text" name="f4" id="f4" class="inline mx-1" />.</p>
	<p>And so the North Wind was obliged to confess that <input type="text" name="f5" id="f5" class="inline mx-1" />.</p>

	<button type="submit">Submit</button>
</form>