Helper skeleton:

<?php

namespace Kanboard\Plugin\MyPlugin\Helper;

use Kanboard\Core\Base;

class MyHelper extends Base
{
    public function doSomething()
    {
        return 'foobar';
    }
}

Register your helper class:

$this->helper->register('myHelper', '\Kanboard\Plugin\MyPlugin\Helper\MyHelper');

Using your helper from a template:

<p>
    <?= $this->myHelper->doSomething() ?>
</p>

Using your helper from another class:

$this->helper->myHelper->doSomething();