1. Download Download SlackBuild:
package.tar.gz

An example: wminfo.tar.gz

2. Extract that package

3. Download Source package and copy that package into extracted SlackBuild package

An example about source package: wminfo-4.2.1.tar.gz

4. Open Terminal and run

su -l

5. Navigate into extracted SlackBuild folder and run the .SlackBuild script.

An example:

sh wminfo.SlackBuild

… or

./wminfo.SlackBuild

6. If the dependecies are ok, the new tgz package can be found from /tmp

7. Installing that new tgz package use installpkg command

An example: installpkg packagename.tgz

You can find SlackBuilds here