1. Package the applet into a JAR file. The applet must be in a JAR file before a certificate can be attached to it. Use the jar JDK utility. If the applet was previously referenced with the help of a codebase attribute in tag, replace the codebase attribute with the archive attribute. The value of the archive attribute is a URL of a JAR file.
2. Create a public/private key pair. The command for this is
keytool is another SDK utility. It will prompt you for a password to your keystore and for the remaining parameters, one of which is alias, whose value is the name of the key. The keystore is a file that contains your public/private key-pairs, and the public-keys of others with whom you exchange information. See the documentation in the above link.
3. Create a certificate for the key you created in the previous step.
Again, keytool will prompt you for a keystore password and remaining parameters. This certificate is now self-signed by you, meaning that it has not been validated by any third party. This is suitable for demo purposes, and may be acceptable to yourself and those who know you because if there is any doubt that the certificate is really yours they can always call you up and ask you for the digest to verify that it is really you and not some impostor that created the certificate. However, if this applet were to be widely distributed, and you wanted it to be accepted by those who do not know you personally, you would certainly want to pay a modest fee to obtain a certificate that is validated by a trusted certificate authority. The procedure for this is straightforward, but beyond the scope of this simple tutorial.
4. Run jarsigner associate this certificate with the JAR file that contains your applet. You will need to give the name of the public key of the certificate you just created. This creates a digest for each file in your JAR and signs them with your private key. These digests or hashes, the public key, and the certificate will all be included in the "WEB-INF" directory of the JAR.
See following websites for further information: