Get the right tool for the job
On a single-sider there’s no wheel alignment to worry about. You need a C-spanner to suit your hub, and a tool to undo the hub-locking fasteners on the rearmost edge of the swingarm. Loosen them, then slide the C-spanner behind the rear sprocket onto the top edge of the large toothed collar.
Rotate the hub to perfection
Pull the C-spanner down gently until you reach the right chain tension. If you over-tighten, move the C-spanner to the underside to go back. Tighten the locking fasteners when you’re done. Doesn’t move? You’ll need to disassemble the hub assembly but don’t use lubricant on the hub/swingarm mating faces.