Use Spackling Knife
Generally, a six-inch speckling knife is recommended for the job. Putting the compound into the joint or space between ceiling and wall is difficult. With the knife you can let the compound feather out without leaving grooves in the middle, which you may not be even able to see.
Put Compound Into Lump
To put the joints compound into the lump put it into the spackling knife and then run it through into the lump. The compound will get into all gaps and grooves, whether or whether not you see them while filling in the joint compound.
After first coat of joint compound, put tapping on the joint between ceiling and wall. Then you can use the knife's edge to push the tape into the joint slightly with your index finger. Be careful and do not push too hard the knife as it can rip through the tap and the purpose of protecting coating can be wasted. If you damage the tape it will not give you the finishing you want out of the joint coat.
Apply Second Coat
The way you used the knife to apply first joint coat, you can do the same again. However, since now you are applying the coat on the tape, you do not have to push your finger too hard on the edge of the knife. Repeat the process by running the compound from one end through the second end, and once done leave it out.
Let Joint Dry Out
After you are done with application of second coat on the joint with the compound, let it dry out. This completes your job and you can see a clear difference of compound that it has given to the joint between ceiling and wall.