add environment setup heading, add mail setup section

docs/INSTALLATION_GUIDE.md

@@ -38,6 +38,8 @@ Install composer dependencies:
 composer update
 ```
 
+## Environment setup
+
 In the [`src/core/`](../src/core/config.php) folder, create a `.env` file with the following contents:
 
 ```php
@@ -54,15 +56,28 @@ BUSINESS_GMAIL=""
 BUSINESS_GMAIL_PASSWORD=""
 ```
 
-Some important notes:
+Update the values assigned to `DB_USERNAME` and `DB_PASSWORD` with your MySQL login details.
+
+> [!IMPORTANT]  
+> If your Apache server is serving from a port other than port 80, include the port number in `PUBLIC_ROOT` (
+> e.g., `http://localhost:443/steamy-sips/public`) .
+
+## Mail setup
 
-- Update the values assigned to `DB_USERNAME` and `DB_PASSWORD` with your MySQL login details.
-- If your Apache server is serving from a port other than the default one, include the port number to `PUBLIC_ROOT` (
-  e.g., `http://localhost:443/steamy-sips/public`) .
-- `BUSINESS_GMAIL` and `BUSINESS_GMAIL_PASSWORD` are the credentials of the Gmail account from which emails will be sent
-  whenever a client places an order. It is recommended to use
-  a [Gmail App password](https://knowledge.workspace.google.com/kb/how-to-create-app-passwords-000009237)
-  for `BUSINESS_GMAIL_PASSWORD` instead of your actual gmail account password.
+1. Choose an email address (gmail account) from which you want emails to be sent.
+2. Go to https://myaccount.google.com/ and login using your desired email.
+3. Search for `2-Step Verification` in the `Security` section and enable it.
+4. Search for `App passwords` in the security section and create an app password with a name of your choice.
+5. In the `src/core/.env` file created earlier, fill `BUSINESS_GMAIL`
+   and `BUSINESS_GMAIL_PASSWORD`. `BUSINESS_GMAIL_PASSWORD` should be set to your app password.
+
+> [!IMPORTANT]  
+> Remove any spaces from your app password when entering it in the `.env` file.
+
+> [!IMPORTANT]  
+> Ensure that you have no firewall or antivirus software (e.g. Avast, McAfee) that block emails. If the mailing service
+> is not working
+> set `$this->mail->SMTPDebug = SMTP::SERVER;` in the `Mailer.php` file and inspect the error.
 
 ## Database setup
 
@@ -116,6 +131,7 @@ In the root directory of the project, run:
 ```bash
 npm install
 ```
+
 ## Autoload setup
 
 Whenever changes are made to the autoload settings in `composer.json`, you must run `composer dump-autoload`.