Before you start, lay a tarp down to make cleanup easier.
Remove growth you don’t want, then thin out spots of thick outer cover. The inner limbs need air and sunlight, so cut back along the branch just above new growth or at the plant base — avoid electric trimmers for this. Trimmers just top the shrub, encouraging new growth near the cut. Cutting farther back will encourage growth along the limb.
Remove any dead limbs near the base. If possible, avoid making the top of the shrub wider than the base — you want sunlight to reach the entire plant. Cut only about 1/3 of the shrub each year. The holes you’re creating will fill in with new growth, making the plant fuller and stronger.