Mix sufficient quantity of mortar, which is a 1 part cement to 2 parts sand with water. Make sure the mixture is not too runny but can stand up with out being crumbly.
Remove any loose mortar, it isn't neccessary to break away mortar that is still set firmly in place unless of couse if it affects the proper reallignment of the brick.
Press fresh mortar into the areas where it were loossened and removed, providing that the brick is stil rigidly in place.
If the entire brick is loose, put sufficient quantity of fresh mortar, as determined by the thickness of what already there and lay the block on top of it.
Check the vertical and horizontal alignment of the brick according to the rest....you may need to tap it slightly in the right places to get best alignment.
It best that you do this block exactly right so you would understand the process so as to make the rest easier.
You can continue to work on another block, with the intention to come back and put morter in the sides when the block has set a bit in the mortar.
If you feel confident enough with the right consistency of the mortar you can do it right away.
Noting that you may disturb the alignment.
Last step is to make sure that each brick is cleaned of all excess mortar and filled other areas as neccessary.
The only tool you would need a is a hand trowel.