Need to upgrade from Tridion 2011 SP1 Sites 9 need more details on what is the upgrade procedure.
From a high-level perspective, here are the main areas you would need to consider. First, look at the Windows Server and Software pre-requisites. If you are considering using the SDL Cloud, you would not need to worry about the CMS pre-requisites, and possibly CD, depending on the Cloud option you choose. However, if you still will host everything on-premise, I hope this short list below gives you an idea to get started with. Also - you will need to upgrade both the Content Delivery and Content Management system at the same time, since version 2011 does not support multiple versions (like a 2011 CMS and Sites 9 Content Delivery system. In the future, upgrades would be easier since you could decide to upgrade one half of the system first, then roll that out, then do the other half...
OK, so the high-level steps are:
Identify the customizations made in the backed CMS (Event System, GUI Extensions, WebServices called from Templates). Any code using VBScript? That would need to be moved to DWT / .Net Templates. Customizations in the Deployer? Make a list of all of these, including the location of source code and a short description of each customization. Generally speaking, this is the source of most of the time spent during an upgrade - porting and upgrading the customizations...
- Update the Windows Server / DB Server software. Upgrade the DB, if needed.
- Run the Tridion CMS Installer and DB Upgrade scripts, going to 2013, then Web 8.5, then Sites 9.
- Install the upgraded customizations and test (this task can be done in parallel with the next ones)
Install Content Delivery / Publishing.
- Publishing has changed a lot in Sites 9, and you will need to allocate some time and energy to setup the Sites 9 way, and to test it. This probably will be your most complicated part of the upgrade.
Install Content Delivery MicroServices
- This is for rendering the Component Links and returning any content from the Broker. These APIs have not changed too much from a functional perspective, so things should work, more or less, as expected.
Finally, I would suggest to do this in a sandbox environment, and install the CMS and CD all on 1 machine to test, so you won't need to worry about any other issues from your environment. I've also found that Azure Windows SQL Servers are fairly responsive and work well. Just remember to turn it off when not using it....
Welcome to stackexchange. docs.sdl.com is obviously a starting point :).
However 2011 SP1 to Sites 9 is a major upgrade (skipping 2013, 2013 SP1, Web 8, Web 8.5). There are numerous strategies to choose from with the newer software. I strongly recommend that you get an upgrade report or assessment performed by SDL in your environment to come up with a high-level recommendations.