Not just ubuntu who has an easy install program using apt-get command. Slackware has one too.
First edit /etc/slackpkg/mirrors
Uncomment one of the closest server from you under the slackware version you use. Mine is Indonesia but the server is not active. So I m using Australian.
Do
#slackpkg update
for updating the repository.
Next time we need to install something do this (in root)
#slackpkg install program
Well done.