If you can use the GeneXus IDE in order to compile and sign, use it, it's the easiest way to do it. Take a look at HowTo: Publish an application in Google Play.
If for some reason you can't, follow these steps:
1) Build your app with GeneXus (do a Rebuild All).
That makes your app be compiled and signed here: <KB Directory>\<Environment Directory>\mobile\Android\<Main Object Name>\bin\<Main Object Name>.apk
is the one that does it.
2) Add to the path environment variable <GeneXus Installation Directory>\Tools\ApacheAnt\bin (for Ant.bat) and <Java JDK bin> directory (for Jarsigner.bat)
3) Save <KB Directory>\<Environment Directory>\mobile\Android\<Main Object Name>\BuildRelease.Bat as your MyBuildRelease.bat and change the content as follows:
call ant release
call jarsigner -verbose -keystore <your keystore filename with full path> -storepass <your store password > -keypass <your key password> bin/<Main Object name>-unsigned.apk alias_name
<Android SDK Directory>\tools\zipalign -v 4 "<KB Directory>\<Environment Directory>\mobile\Android\<Main Object Name>\bin\<Main Object Name>-unsigned.apk" "<KB Directory>\<Environment Directory>\mobile\Android\<Main Object Name>\bin\<Main Object Name>.apk"
4) Take a look at <KB Directory>\<Environment Directory>\mobile\Android\<Main Object Name>AndroidManifest.xml to see if VersionCode and VersionName are OK and if Android Application Permissions are OK.
5) Run MyBuildRelease.bat and get your apk: <KB Directory>\<Environment Directory>\mobile\Android\<Main Object Name>\bin\<Main Object Name>.apk