I don't have experience in doing this. I got my iphone second hand and is already in jail break. I believe my phone is in version 3 something. My son has an Ipad two that we reset to factory settings since my husband wanted to restart "Clash of Clan" and we cannot find an option. Then he encourages me to update the OS of my Iphone 3GS. With me no idea how to do jail break I just updated my iphone OS using Itunes. And take note I did not even created a back up (aw! huhuhu). Update went through with no error but now my phone no longer boots. I then searched in google overnight of what I need. I've done so many things, no idea of what jail break really is, no back ground at all.
I'm really not sure of whats the right step by step process that makes it successful but let me post here what I downloaded and what I think makes it successful. First I downloaded Redsnow 9.15b3 and iphone2,1_6.0 IPSW. Don't download 1_6.1.3 thats what I've searched online.
Then I followed this link (you can click here) on how to do a jailbreak. During this process I encountered an error on the just boot, it hanged on reboot process. What I did is reset the phone by holding down both the power and home button and do the just boot again. (NOTE: Everytime you do a reboot you need to run Just Boot in redsnow). After the jail break next problem I encounter is I don't have signal. It shows no service or Itunes shows no sim. What I did is follow this link (click here). I restore my iphone first using Itunes, then do jail break again but since it already shows Bootrom of 6.1.5 I run jail break with check on Downgrade Baseband. No need to install Evasion, just install cydia and ultrasnow, don't install ultrasnow fix. After this I run just boot and put my sim card back. To my surprise it shows Globe Signal, yey!!! Bad thing though I lost everything, my notes, pics, contacts and apps. Good thing though is I can now install apps that doesn't work before with my old OS like facebook, candy crush, viber etc. Well it was a good experience, I remember I paid P800 to jailbreak my phone before. Now I can do it on my own. Be careful doing this since it can brick your iphone.