Go2Linux posted a tutorial about building packages from sources for Slackware with slackbuilds.org

Slackbuilds is the best option to install software in Slackware, software that is not in the official slackware repository.
How to use Slackbuilds.org
I will use htop to show you how to use slackbuilds.org
Go to slackbuilds.org
Using the search box, look for htop

  Build packages from sources for Slackware with slackbuilds.org