You can only do this after downloading the Lion operating system from app store and before installing it. Once you install Apple deletes the file.
So do this after downloading and before installing.
Making a bootable Lion disk will come in handy in future when you want to reinstall Lion without downloading it again.
The only way to clean install Lion is by using snow leopard.
So find the free snow leopard dvd or usb recovery disk you received and install snow leopard again. This time when at the installation screen make sure you erase the hard disk by using disk utility. Which can be found in the menu bar during installation.
Once you install do a software update. You should get mac os updates which will be around 1 GB (the size of the update will depend on which version of snow leopard you have). Once you download and install that pop in your newly created Lion usb or dvd and install Lion on this clean version of snow leopard.
If you are doing this only on one machine you don’t even need to make the bootable disk. Just clean install snow leopard and do the updates and download and install Lion from app store.
Why would you want to make a clean install ? Thats a good question
I have had my mac for the last three years and only upgraded to the new os so far. I install every new program that comes around so. So there is lot of junk on the machine now, so i wanted a clean start