Patience in battle. Much patience. Learn from your deaths. Beat all the bosses. You go back to the dream at each lantern and you purchase items or level your character up with them.
Just to add: you need at least 1 insight to activate the Doll at the Hunter's Dream, who levels you up. Here's a rundown from the wiki that lists how you gain insight:
Insight is gained as the player progresses through the game, defeats enemies, and finds new areas.

Entering a boss area for the first time grants +1 Insight.

Entering Iosefka's Clinic through the back door grants +1 Insight.

Entering the Abandoned Old Workshop grants +2 Insight.

Entering Yahar'gul, Unseen Village after being killed by a Death Dealer grants +1 Insight.

Entering the Cainhurst Castle grounds grants +2 Insight.

Defeating a boss grants +3 Insight.

Defeating a boss in another player's world grants +1 Insight.

Consuming a Madman's Knowledge grants +1 Insight.

Consuming a Great One's Wisdom grants +2 Insight.

Consuming One Third of Umbilical Cord grants +3 Insight.

Sending an NPC to Iosefka's Clinic or Oedon Chapel grants +1 Insight.

Killing an adversarial host player grants +1 Insight.
Easiest way is to just enter the Mad Cleric boss fight, or find a Madman's Knowledge. The downside with the boss fight is, you will probably die and thus lose any souls that you have.