There are two basic methods for bonding a portland cement based product to existing concrete;
The chemical approach involves bonding agents that aid in bonding new concrete products to old. Products like Sakrete Top'n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent.
Products like Sakrete Sand Mix and Sakrete Fast Setting Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder & Fortifier.
When you use a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. Just as in the process described above, after the bonder has become tacky apply a scratch coat and then apply the repair material.
The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. In the case of this material, it can be used as a brush on or as an admixture. See the product technical datasheet for more information regarding use as an admixture.