Well I have created a web-based POS to take order. My question is how do I create a Java-Applet;
You cannot do that for security reasons. If you could, applets would already have become notorious for printing 10+ pages of 'special offers' when you visit unscrupulous web sites.
OTOH, if the client is willing to accept one prompt at applet start-up, you could digitally sign the code.
There is a project that does HTML printing using HTML5 to render the contents to a PNG and Java to print directly, exactly as described in the original post. This project is called "qz-print" (previously called "jzebra") and it offers the digital signature in both self-signed (free) and trusted-signed (at a premium).
It also uses the signed JNLP files as Andrew Thompson has illustrated. (Thanks Andrew, your contributions to Java as a whole have been a great help to Java developers around the world).
https://code.google.com/p/jzebra/