Safety first! When working with any refrigerant there is the potential of frostbite if the refrigerant comes in contact with eyes, skin, and clothing. Proper PPE is required. Always use in a well-ventilated area. Manifold and hoses must be able to accommodate the higher-pressure range of R-410a.
Many of us have been taught when adding refrigerant to a system, this should be done by adding vapor refrigerant into the suction line. Vapor is recommended to insure one does not flood the compressor with liquid and potentially damaging the compressor.
Unlike R-22 where refrigerant can be added in either the liquid or vaper state, blends like R-410a MUST be added in the liquid state only. In liquid form, R410a 
refrigerant is a 50/50 blend of R-32 and R-125. What needs to be noted is that R-32 and R-125 have different boiling points and condensation points. The vapor in the drum will have different proportions of refrigerants compared to liquid.
When charging a blended refrigerant with liquid, the ratio will not change, but if charging with vapor, the ratio can be very different. As one charges with vapor, the refrigerant blend will separate, and the vapor ratio can change drastically. When charging with vapor, there is no way to tell what ratio of refrigerant one is adding to the system.
Depending on how much vaper is added, how a system operates can be compromised and unexpected results may be seen. In small amounts, adding R-410a in the vapor mode, may not have much of an effect on how the refrigerant system operates.
When R-410A was first introduced, the cylinder had an internal dip tube which allowed the feeding of liquid refrigerant when the cylinder was in an upright position. Manufactures today do not utilize a dip tube. Always check manufacture instruction for cylinder orientation for liquid charging.
Adding Liquid Refrigerant into the suction line with the compressor operating you MUST flash or throttle the refrigerant. This must be done; otherwise liquid refrigerant may enter the compressor (slugging). Slugging can cause the oil to be washed out of the compressor. Compressing liquid refrigerant can cause damage to valves or scroll plates.
Many manifolds have sight glasses incorporated into them. This is done so the refrigerant can 
be flashed and monitored while charging.
Allow sufficient time for the refrigerant system to stabilize before adjusting the charge. It is very easy to overcharge or remove too much refrigerant when one is in a hurry. Some higher efficiency systems and inverters recommend up to 15 minutes for stabilization of refrigerant before adjusting charge.
If liquid refrigerant is added to the suction line when the system is not running, there is the potential of the compressor being flooded on start up. In some cases, the compressor will not start. When the system is not running, liquid refrigerant should only be added to the liquid line.
Make sure you understand how the refrigerant can be added to a system. Some refrigerants must be added only in the liquid mode and must be throttled to avoid slugging the compressor. Take your time allowing for the refrigerant system to stabilized before adjusting again. Refrigerant charge is critical to operation of a system.