So after I download all the game files, I still can't install the game.

That is why? You get some error message or nothing happens? Some more information would be really helpfull.

my bad .
There are two error messages :
Message number one : installer files missing . All files were downloaded and saved into singe folder . I've made sure to check that installer uses the folder as its directory , set in the options .
Message number two occurs when i try to select file when installer prompts to select CD1 for installation . I chose file labeled as 1 . System cannot read it and I get message to abort the installation .

O.K. So you have downloaded all 7 files (1 exe-file and 6 bin-files). This is strange. Perhaps one file got corrupted during the download. Did you use your browser, GOG Downloader or GOG Galaxy to download the game?

got all seven files. I've checked that I've downloaded everything .
browser download . should i try the official downloader ?
Im going to delete the files and try again tomorrow , i guess. Thanks .

For me it seems like one ore more files weren't downloaded correctly. Best case would be if it is only the exe-file (smallest file). You could try to redownload the exe-file and double click it again.
You could try the GOG Downloader (https://www.gog.com/downloader).
If you use Galaxy be aware that you only get install files for a later installation if you use the backup function (more button). Otherwise it only downloads the game and install it.