Using SiaStream - the Settings
Get to your Settings from the second tab across the top.
FUSE
Set your FUSE Mount Path. This is where you want the Sia FUSE drive to be mounted on your computer. The FUSE mount is what allows your media player to see the media stored on Sia, so make sure you point Plex or your preferred player to this location.

Media Paths
Media Root Directory. This needs to be pointed to the local folder where all your media is. SiaStream will automatically monitor this location and upload the contents to the Sia network. You still do what you've always done, when you get new media, just drop it in here.
Sia Path. This is where Sia stores all of its important files, like the blockchain and your wallet.

Budget and Spending
Sia is based on an open marketplace, where hosts are competing for your data and driving prices down.
Target Price (per TB/month). Set how many Siacoins you want to spend per TB/month. You'll get more familiar with pricing as you use the app, so if you're not sure what to put here you can click Use Recommended Price. This number is a middle of the road estimate based on recent prices paid for storage, pulled from siastats.info, an independent third party.
Storage Amount (in TB). Set this to tell SiaStream how much storage you think you'll want each month. You only ever pay for what you use, but this helps the app get a headstart in case you need a lot of space.

Additional Settings
Remove media after upload. This turns SiaStream into the sole storage location for your media, deleting it after it gets uploaded to the network.