Cornstarch dissolved in COLD water--not much water...you don't want to dilute the lemon juice any more than you have to, but you want liquid, not paste. A teaspoon in half a cup of water should do it.

Then heat the lemon juice, but not to boiling, just hot...stir in the cornstarch a LITTLE at a time--just teaspoons at a time--till you reach the desired thickness.

It's not a disaster if you don't get it right the first time, but unless you want to go through a lot of lemon juice working on just the right balance, you might want to practice thickening plain water first till you get the hang of it.