Direct answer: Siofra River is reached by a small domed building called the Siofra River Well, located just north-east of the Minor Erdtree in the Mistwood, in eastern Limgrave. It costs nothing, needs no key, and no quest. Step on the lift, ride it down twice, and you are in.
The approach
Ride north-east from the Agheel Lake area through the Mistwood until you see the Minor Erdtree, then keep going past it. The well is the unremarkable stone building on the far side.
The one thing to watch for on the way in is the bears. Large ones roam this forest, and they are a genuinely unpleasant surprise for a character built around casting or light armour. They hit hard and they do not stagger easily. If you are under-levelled for the area, the bears are the reason people think Siofra is harder than it is.
Descending
Inside is a lift. Take it all the way down, and when it stops, activate the Siofra River Well Depths Site of Grace immediately — it is just to the right of the lift platform. This matters more than it sounds: the region is large, the lighting is dark, and fast travel underground works exactly as it does on the surface, so a grace you have not activated is a long ride back.
Sites of Grace you will want in Siofra: Siofra River Well Depths, Siofra River Bank, Worshippers’ Woods, Below the Well, and Ancestral Woods. Collect them as you pass rather than deliberately backtracking later.
What survives down here
More than you would expect. Torrent works underground. Fast travel works. Your flasks are replenished when you clear a group of enemies, which is a meaningful help in a region that runs long. Almost nothing changes except the sky, which is the point.
There is also a Map Fragment on a corpse near the stairs up to the Hallowhorn Grounds, east of the Siofra River Bank grace. Pick it up. Siofra is big enough that the map genuinely changes how you explore it.
One thing to know before you commit
Nokron, Eternal City is not available on your first visit. It opens when a star falls on Limgrave, which happens after you defeat Starscourge Radahn. Nokron holds the Fingerslayer Blade, which is the critical path for Ranni’s questline. So if you came down here hoping to start that questline, you have arrived early — and that is fine, because Siofra has plenty for you to do first.
Evidence status
Research-reviewed. The well’s position relative to the Minor Erdtree, the two-stage descent, the Well Depths grace position, the confirmed Sites of Grace list, the map fragment location, the underground flask and fast travel behaviour, and the Radahn gate on Nokron were each cross-checked against multiple independent references. Not gameplay-verified on camera.