To install ATag, first unpack the ATag distribution file.
In the root of the distribution directory, you will find 3 files: ATag.jar, atag.tld, atag.js.
Place ATag.jar in your webapp/WEB-INF/lib/
Place atag.tld in your webapp/WEB-INF/
Place atag.js somewhere in your webapp so that you can refer to it from you html code.
Next, you'll need to declare the ATagFilter in your webapp/WEB-INF/web.xml, and map it so that it intercepts requests to your webapp controller. This filter should not be placed in the path of JSP views.
ATag has a runtime dependency on commons-logging and commons-beanutils, so if these libs are already on your classpath, then you're good to go, otherwise copy the jars in the lib/ directory of ATag distribution to your webapp/WEB-INF/lib/.
See package specification of the taglib package in ATag API docs for more information.