To activate debug mode in Magento 1.9 we need access to the hosting either via ftp or through the plesk or cpanel panel.

The first thing is to log in to our website and go to the following path System -> Cache Management

In this view, we select all types of cache and go to the dropdown and select Disable, finally we click on Submit.

The next step is to go to the System menu -> Index Management

In this view we click on select all and in actions Reindex Data we finally click on Submit.

Then, we will go to System -> Tools -> Compilation.

If the compilation process is enabled, we will disable it by clicking on Disable, it comes disabled by default.

Once this is done, we go to System -> Configuration.
In the window that opens, we will look for Advanced -> Developer in the left-hand menu.

In the view that opens, we will expand the Log Settings option and in the enabled option, we will set the option to Yes, we will click on Save Configuration.

We continue in this window and go to where it says Current Configuration Scope, there we must select Main Website, now in the Debug tab, we have to change Template Path Hints to Yes.

Now we are going to modify some files in our hosting.

In the index.php file we have to modify the following line: "#ini_set('display_errors', 1);".

Now we must rename the local.xml.sample file to local.xml, which is in the errors folder.

With this we already have our Magento 1.9 in debug mode.