The sauce gets made first, then split. One cup marinates the chicken, the rest becomes the glaze. That split is what makes this work and it is also the part where a food safety mistake is easy to make.
Four servings, and two hours of the total time is the chicken sitting in the fridge.
This is worth being explicit about, because the recipe implies it without saying it.
You make the full batch of sauce, cool it, and pour one cup over the raw chicken to marinate. The remaining sauce is reserved separately and never touches raw meat.
That reserved portion is what goes into the skillet at the end. The cup the chicken sat in is discarded, because it has been in contact with raw poultry for two hours.
Do not pour it back into the pan to stretch the sauce. Make the full batch, use one cup for marinating, and you will have plenty left.
Ten to fifteen minutes off the heat.
Hot sauce poured over raw chicken starts cooking the outside while the inside stays raw, which is both a texture problem and a food safety one, since the meat sits in a warm environment rather than a cold one.
Cooled, it goes into the fridge with the chicken and everything stays at a safe temperature for the two hours.
Rice vinegar, lemon juice and orange juice, against a full cup of packed brown sugar.

That much sugar sounds extreme and it is what makes the sauce sticky and glossy in the way takeout orange chicken is. Sugar concentrates as the sauce reduces and it is what makes it cling to the chicken rather than running off.
The three acids stop it being simply sweet. Rice vinegar is mild and slightly sweet itself. Lemon is sharp. Orange juice is neither, mostly contributing flavour, with the zest doing the heavier lifting on that front.
A tablespoon of grated orange zest is where most of the orange flavour actually comes from, since the oils in the peel survive cooking where juice largely does not.
Flour, salt and pepper in a resealable bag, then the drained chicken added and shaken.
A bag coats small cubes far more evenly than tossing in a bowl does, and it uses less flour. It also keeps your hands out of it.
Shake off the excess as you transfer the chicken to the pan. Loose flour burns in the oil and makes the sauce cloudy later.
Cooked in olive oil over medium heat until browned on both sides, then onto a paper towel lined plate and covered with foil.
The chicken is not cooked through at this stage. It goes back into the simmering sauce for five more minutes, and taking it to done now means overcooked chicken at the end.
Then wipe the skillet out. The bits of loose flour left behind would go bitter and cloud the sauce.
Three tablespoons mixed with two tablespoons of water until completely smooth, then stirred into the boiling sauce.
Dry cornstarch in hot liquid seizes into lumps immediately. Mixed with cold water first it disperses evenly.
The sauce has to be boiling for cornstarch to reach full thickening power. Add it to a warm sauce and it will not thicken properly.
Heat reduced to medium-low, chicken added, simmered until cooked through.
Chicken should reach an internal temperature of 165F. Half inch cubes get there quickly, and five minutes in a simmering sauce is enough, but a thermometer through the largest piece is the check.
Over rice, which is what the sauce is designed for.

Around 445 calories and 18 grams of protein per serving. Sodium sits at 763mg, mostly from the soy sauce.
The sauce can be made up to three days ahead and refrigerated, which removes the cooling wait on the day. The chicken can marinate overnight rather than two hours, and the flavour is better for it.
This version pan fries in three tablespoons of oil, which browns the coating without making it properly crisp. For something closer to takeout, use more oil and a higher heat, or shallow fry the cubes in about half an inch of oil for two minutes a side before saucing them.
A sticky sweet orange chicken with rice vinegar, orange zest and brown sugar, served over rice.
The cup of sauce used to marinate the raw chicken must be discarded and never returned to the pan. The remaining sauce is reserved separately before the chicken touches it and is what becomes the glaze. Cool the sauce 10 to 15 minutes before pouring it over the chicken. Chicken should reach an internal temperature of 165F. Mix the cornstarch with cold water into a slurry before adding it, and add it to a boiling sauce, since cornstarch needs boiling point to thicken fully. Wipe the skillet after browning the chicken so loose flour does not cloud the sauce. Nutrition per serving from source: 445 calories, 69g carbohydrates, 18g protein, 11g fat, 2g saturated fat, 34mg cholesterol, 763mg sodium, 1g fiber, 37g sugar.