The toilet cistern had been leaking for quite a while, and tonight, I took on the task of fixing it.
Initially, the difficult part was my struggle of unscrewing the plastic washer nut, as there were no spanner big enough to dislodge it from its location underneath the cistern itself. Eventually, I figured out I had previously removed it using my bare hands, together with a piece of cloth.
And once I remembered that part, it wasn't too difficult to unscrew the washer. The rest of the fix were quite mechanical. Just unscrew and screw up everything properly.
In case some day I need to fix the toilet cistern again, I would remember what I did.