The manual is clear enough. Please specify what part you get stuck with after you work through it, I'd suggest:
https://developer.android.com/studio/publish/app-signing.html
Okay, a small overview without reference or eclipse around, so leave some space for errors, but it works like this
Also, from the link:
Compile and sign with Eclipse ADT
If you are using Eclipse with the ADT plugin, you can use the Export Wizard to export a signed .apk (and even create a new keystore, if necessary). The Export Wizard performs all the interaction with the Keytool and Jarsigner for you, which allows you to sign the package using a GUI instead of performing the manual procedures to compile, sign, and align, as discussed above. Once the wizard has compiled and signed your package, it will also perform package alignment with zip align. Because the Export Wizard uses both Keytool and Jarsigner, you should ensure that they are accessible on your computer, as described above in the Basic Setup for Signing.
To create a signed and aligned .apk in Eclipse:
- Select the project in the Package Explorer and select File > Export.
Open the Android folder, select Export Android Application, and click Next.
The Export Android Application wizard now starts, which will guide you through the process of signing your application, including steps for selecting the private key with which to sign the .apk (or creating a new keystore and private key).
- Complete the Export Wizard and your application will be compiled, signed, aligned, and ready for distribution.