Assuming we have developed the following extension to Magento admin panel:

app/code/local/Engineering/Custommodule/

And we can actually use it in the admin panel, but when we would like to set the permissions - we go to System -> Permissions -> Users, and try to give the right permissions to our users.

However, it doesn't seem to work, and the non-administrator users will not be able to view the extension (lack of permissions).

To solve this, we would create a new adminhtml.xml file in app/code/local/Engineering/Custommodule/etc/

Inside the file, we will have the following code:

<?xml version="1.0" encoding="UTF-8"?>
<config>
	<menu>
		<cms> //change to whatever tab you want
			<children>
				<custommodule translate="title" module="custommodule">
					<title>Custom Module</title>
					<action>custommodule/adminhtml_custommodule</action>
				</custommodule>
			</children>
		</cms>
	</menu>
	<acl>
		<resources>
			<admin>
				<children>
					<cms>
						<children>
							<custommodule translate="title">
								<title>Custom Module</title>
								<sort_order>0</sort_order>
							</custommodule>
						</children>
					</cms>
				</children>
			</admin>
		</resources>
	</acl>
</config>

Overriding a core model in Magento can be done by creating a Magento extension:

Let's say we would like to override app/code/core/Mage/Newsletter/Model/Subscriber.php

1. Create the company directory in local (app/code/local/Engineering).

2. Create the extension directory inside (app/code/local/Engineering/Newsletter)

3. Create model & etc directories inside (app/code/local/Engineering/Newsletter/model & app/code/local/Engineering/Newsletter/etc).

model - what we would like to rewrite

etc - the necessary configuration to tell Magento we're overriding a core model

4. Copy the model there (Subscriber.php).

5. Change the copied-model's class name to the following:  class Engineering_Newsletter_Model_Subscriber extends Mage_Newsletter_Model_Subscriber

6. Create a config.xml file in app/code/local/Engineering/Newsletter/etc

7. Place the following inside the config.xml:

<?xml version="1.0"?>
<config>
    <modules>
        <Engineering_Newsletter>
            <version>0.1.0</version>
        </Engineering_Newsletter>
    </modules>
    <frontend>
        <routers>
            <newsletter>
                <args>
                    <modules>
                      <Engineering_Newsletter before="Mage_Newsletter">
                         Engineering_Newsletter
                      </Engineering_Newsletter>
                   </modules>
                </args>
            </newsletter>
        </routers>
    </frontend>
    <global>
        <models>
            <newsletter>
                <rewrite>
                    <subscriber>Engineering_Newsletter_Model_Subscriber</subscriber>
                </rewrite>
            </newsletter>
        </models>
   </global>
</config>

* Bear in mind: the xml file is case-sensitive, and you'd better stick to the conventions (capitals wherever needed) - that's where alot of beginners waste hours of debugging, trying to understand where their mistake is.

Repeat after me. we never, ever, ever, ever - change Magento's core files (app/code/core).

Why? lots of reasons. the most important one is, we would like to upgrade our Magento system at some point.

The best of practices would be to create a directory in app/code/local named Mage (app/code/local/Mage).

Now copy the core file to the exact same directory hierarchy (create the same hierarchy), on local.

Which means, if the file we would like to copy is at app/code/core/Mage/catalog/controllers/IndexController.php, we would copy it to app/code/local/Mage/catalog/controllers/IndexController.php.

Blocks, controllers, helpers - can be copied like this without a problem.

Models, however, require you to develop a Magento extension with slight modifications.