UO: Serpent Isle uses a customized version of the 7.0.20.0 client, including art and music. The latest version of this client can always be found here.
The current UOSI client version is 0.6
UO: SI Client Downloads- Mega Link
Mirror #1- MediaFire Link
If you already have a client package, you can instead download the patch. This only contains the changed files and is much smaller.
You will also need a UO Assistant to connect. UO: Serpent Isle proudly endorses ClassicUO for its smooth framerates and modern features, however, any assistant should work.
Download ClassicUO here.
Download UORazor here.
Download UOSteam here.
INSTALLATION INSTRUCTIONS
Extract the UOSI CLIENT zip to a convenient location.
If you have downloaded a patch, extract the contents of the folder into your existing UOSI Client folder. Overwrite all files.
ClassicUO
Once you’ve completed the installation steps, open your settings.json file in notepad or notepad++. Change the value of “ip” to “play.uoserpentisle.com” and the value of “port” to 2593. You will also need to change “ultimaonlinedirectory” to point at the extracted UOSI CLIENT files. Finally, change the value of “clientversion” to “7.0.20.0” and save the file.
Open ClassicUO and click the “Advanced” tab on Razor window. Ensure “Negotiate features with server” is checked. You can now continue to the “Getting Started” instructions below.
UORazor
Under “Client Options,” browse to locate “client.exe” from the package. Then, set “UO Data Directory” to the extracted location.
The “Server” is play.uoserpentisle.com and the port is 2593.
Ensure “Patch client encryption” is checked and “Use OSI Encryption” is unchecked, then press “Okay” to start.
The game client and the Razor assistant window will now open. In the Razor assistant window, select the “Options” tab, then select “Negotiate features with server,” or you will not be able to stay connected. You can now continue to the “Getting Started” instructions below.
UOSteam
Configure UOSteam to utilize “client.exe” from the package, and set the directory to the extracted location.
The “shard” is play.uoserpentisle.com and the port is 2593.
Ensure “Remove encryption” is checked and that all other checkmarks are cleared, then press “Start.”
The game client and the UOS assistant window will now open. In the UOS assistant window, be sure to check “Negotiate features with server,” or you will not be able to stay connected. You can now continue to the “Getting Started” instructions below.
GETTING STARTED
Login with your desired username and password – this will create a new account. Once you create your character, you will start at the Serpent Pillar Expedition Camp and begin your journey to the Serpent Isle.
Once you enter the game, open the Options menu from your Paperdoll and go to “Sound and Music.” This is usually the first screen that opens. Make sure “Play footstep sounds” is unchecked. 
The server plays sounds for surfaces that clash with the default footstep sound. This will prevent that.
FAQ
Q: Why do I get kicked shortly after I’ve joined with a warning about not having the most recent version?
A: Make sure “Negotiate features with server” is checked in your UO assistant settings. See above for specifics for each assistant.
Q: What is that awful sound I hear when I’m walking around?
A: The recent patch added footstep sounds to specific surfaces. These clash with the default footstep sounds if this option is enabled. Please go to Options, “Sound and Music,” and uncheck “Play footstep sounds” to resolve this problem.
Q: My issue isn’t listed here, what now?
A: Come to the Discord and we’ll sort it out.