I tried to install openbox by running slackpkg install openbox, which I knew would not work. I went to SlackBuilds and in the Howto it says to download the archive and also the source code for the install. Now, what if I download a tarbell which does not have source like steam? Would I still be able to install it?

you can always download binary package (and source) from AlienBOB's repository: http://www.slackware.com/~alien/slac...s/steamclient/

Alien bob normally puts a "dependencies" txt file in the package directory. Just pull the whole directly down to your hard drive and take a moment to read it, it will save you a lot of trouble.
Slackbuilds lists dependencies on the page where you pull down the packages. Similarly, make sure you've got what you need to install the pkg before you try to install it and with Slackbuilds be careful you're downloading the right binaries (32 vs 64 bit) Alien splits the directories so it's hard to make that mistake
Openbox will work like a charm if you read and follow the directions carefully.

Sorry, let me answer your question.

Yes,
Download all the files in the directory containing your package.
Do an md5sum check to make sure you have the uncorrupted source in a tarball. If you need help with that, post back
Go to a terminal, go to that directory and use a user who can "sudo" to root permissions, preface each of these commands with "sudo".
"tar xvf" <sudo tar xvf>the file with only the package name and not the version number, it will create a directory beneath it. For example "openbox.tar.gz" will create a directory called "openbox". Move all the files related to openbox into that directory and then CD into it.
Read all READMEs' and text files. If you have a question about what you're reading, ask.
Look for a file named "openbox.Slackbuild". It will be in the directory you downloaded and moved.
You have to change the permissions on that file so
"sudo chmod 770 openbox.Slackbuild"
Then you just type "sudo ./configure openbox.Slackbuild" and it will build your package. The build can be a little intimidating when you first start doing this, eventually it will make you sleepy watching it.
When it's done it will tell you it copied the installable package into /tmp/sBO/<packagename>".
"installpkg /tmp/sBO/<packagname>" and you watch the script run until it pops out with "package "packagename" installed."
You don't have to type "ldconfig" at the end but it's kind of like throwing salt over your shoulder.
You should be golden.
Aliens install basically the same way. except he has the source built if you just want the defaults. Just download everything and follow the steps up to "./configure". With aliens stuff you just move it to /tmp and type "sudo installpkg packagename" and it will install
I'd pick a night I had some time and a package that I didn't care much about and practice with it. It sounds harder than it is, after you've done a couple it will be second nature to you. Almost everyone who has "Slackware" under their name can help you with this so you can always get a relatively quick answer from here.
The advantage to this system over the automated one is you'll notice as you get better at this that you can control a lot of the functionality of the packages with switches you insert when you configure them. You get a much better level of control over what you're installing and how it's going to work than when you use an automated package handler.