Cannot figure out how to add a read more link to a manual excerpt. Please help

There are several options you have. You can write your own excerpt, or just use the tools which WordPress offers to you.

There are three options you have:

  1. Use WordPress core function the_excerpt();
  2. Use WordPress core function get_the_excerpt();
  3. Use WordPress filter wp_trim_excerpt();

The easiest and propably most fitting for you is the first one.

<?php

    // WordPress template function
    the_excerpt();

 ?>

Excerpt from official WordPress docs:

Displays the excerpt of the current post after applying several filters to it including auto-p formatting which turns double line-breaks into HTML paragraphs. It uses get_the_excerpt() to first generate a trimmed-down version of the full post content should there not be an explicit excerpt for the post.

Now, you want to add a “read more custom link”.

Basically, the first step is to add a WordPress filter to the excerpt.
The filter we are using is called excerpt_more.

<?php
    // Mechanism to add a filter to something
    function functionname() {
        // Custom code to filter/modify

    }
    add_filter('filtername', 'functionname');
?>

The next step is to call the excerpt function where you want it.

 <?php
    // Put these 3 lines of code in your functions.php
    function wpdocs_excerpt_more( $more ) {
        return '<a href="'.get_the_permalink().'">Permalink</a>';
    }
    add_filter( 'excerpt_more', 'wpdocs_excerpt_more' );

    // This function call needs to be placed within "the loop" of your home.php
    the_excerpt();
 ?>

The function “the_excerpt” needs to be called within the loop. So, make sure you are adding it in the correct place.