How can I make next/previous buttons to navigate my projects?

In the layout_projects.php file of your theme, place the nextProject() and prevProject() template tags where you would like them to be displayed.

Your layout_projects.php may look like this then:

<?php include_once "header.php"; ?>
<div id="project_main">
    <?php echo projectView(); ?>
</div>
<div id="navigation">
    <a href="<?php echo prevProject(); ?>">&larr; Prev</a> | <a href="<?php echo nextProject(); ?>">Next &rarr;</a>
</di>
<?php include_once "footer.php"; ?>