[ubuntu] How do I install scilab 6.0 from a tar.gz file step by step?

Why exactly do you need to install it using an archive? It is in the repos, though not that version; 14.04 has 5.5.0-2; 16.04 has version 5.5.2-2.
Do you really need version 6.0.0 or will 5.5.2-2 do all you need?

It is not really possible to tell you how to install a package from an archive without knowing where the archive came from and what files are included in the archive, which could be source code files or could be an executable, or even a .deb package file. I was going to have a look myself but when I saw the download was 325MB decided that I would rather not as I have no possible need for it.

So extract the archive scilab-6.0.0-beta-2.bin.linux-x86_64.tar.gz which will normally produce a folder called scilab-6.0.0-beta-2.bin.linux-x86_64 and see what is in that folder. There should normally be an install or README file with instructions. If it is source code you will probably need to install the build-essentials package in order to be able to configure, make and install the application.

See if that helps; if it is all gobbledegook to you come back again after seeing what the archive contains and we'll try to help more.