Step 1:

Click the "Start" button

Step 2:

Type "cmd" in the search box

Step 3:

The program "cmd.exe" should now appear, select it

Step 4:

The Microsoft Command Prompt will now open

Step 5:

Type "telnet {domain name} {port number}" and press enter, eg "telnet cwcs.co.uk 80"

Step 6:

Depending on the port you telnet the result returned will be different, here is an example of some common ports and their results. 

Port 110 - POP3

Port 25 - SMTP

Port 3306 - MySQL

Port 21 - FTP

Step 7:

To exit telnet you need to hold down "ctrl" and press "]" (the key has both the } and ] brackets). This will drop you out of the telnet connection, you then need to type "quit" and press enter