I have a 3Gs with 5.0.1 factory unlocked , new bootrom

I want to downgrade to 4.X

Is there any tutorial to follow (I tried find it in the forum with no help) .Google point me a lot of blogs with older process that end in multiple errors.

Thanks in advance

You can only downgrade if you have SHSH recorded for the version of iOS you want to downgrade to. Click the SHSH link in my signature if you don't know what they are, and use TinyUmbrella to determine what SHSH are on file with Cydia.

If you don't have SHSH, you cannot downgrade, period. Find out which you have and let us know, then we can help better.

I have saved manually in my PC for 4.1 .
Tiny says I have 4.1 , 4.2.1 , 4.3 , 4.3.3 , 4.3.5 in general tab

Thanks again

OK, use TinyUmbrella to download the SHSH files locally to your computer if you haven't already. Go to iClarified and download the .IPSW corresponding to the version you want to downgrade to (I recommend 4.3.3, older versions are missing features and newer ones don't add anything important), then start the TinyUmbrella TSS server, and shift-restore and point to the 4.3.3 .IPSW. You may end up with a -1016 error or -1015 error, in which case, click the 'Kick device out of recovery' button in TinyUmbrella. You should then be downgraded to 4.3.3 and good to go.

Make 100% sure that 'set hosts to cydia on exit' is NOT checked, and also check your hosts file to ensure that you have removed any lines that point to 'gs.apple.com'

Follow step by step
shift - restore , point IPSW , start restoring
Error (20) and recovery mode .
Close iTunes
Exit recovery mode in Tiny
iPhone still in 5.0.1 working fine

Cant pass (20) error !!! any help ?

If you're getting Error 20, attempt the restore again in DFU mode. If that doesn't work, use iReb/redsn0w to put the phone into 'Pwned DFU' mode and try again. One of those two should resolve your error.