I have the empty grand soul gem and the soul trap spell and the light is on the alter . Although I can drop the gem I can`t place it on or in the alter nor can I equip it because it`s empty
I can grab it but then what. Any help ? please

During the shade of the revanent you activate the alter (it's a container like a chest) and place the grand soul gem inside than cast a soul trap spell on the altar then retrieve your new black soul gem.

I've done it before, though, and it never worked for me :/

Of course, I don't remember if it was empty, or had some soul in them *shrugs*

http://www.uesp.net/wiki/Oblivion:Black_Soul_Gem

when I activate the alter all I get is a screen showing what`s in it , obviously It`s empty at the moment so how do I tranfer the grand soul gem from my inventory to the alter inventory screen ???

Ohhh, I understand you now although its hard to explain how the inventory screen works without a visual aid (btw how did you get that far in the game without figuring out how to put your own stuff in containers?).

At the bottom of the inventory screen after you select the altar there's a bag icon next to the take all button, click on that icon and it will take you to your inventory where you find the grand soul gem; then click on the soul gem to drop it into the altar and then finish the rest of the steps to create your black soul gem.

thanks a lot it`s easy when you know how isn`t it