I usually use my Tom's gardening tongs/claws for planting, you can get the roots real deep into the substrate while staying dry.
Try to remove as much of the rock wool as possible. A good 20:1 bleach dip before setting is good too.
Note: this is generally speaking since all plants have individual planting preferences.
For rooted plants: cut the tips of the roots to induce new growth [don't cut the roots of anubias]. Make a hole, insert (deep), cover, and then slightly pull up on it [Vallis and sags should have the upper portion of the crown visible].
For bulbs and rhizomes: Again, cut some of the roots. Some of the bulb should be protruding from the substrate [banana plants should be more out than in]. Plant rhizomes at an angle so that while still deep, the sprouting end, or "eyes", of the tuber will not be restricted.
For stem plants: If possible try to plant them in groups with each stem having its own hole -this will curtail the spreading of rotting roots. Take off some of the lower leaves to avoid them from being covered -new roots will sprout from these nodes anyway.
Fern species and some bolbitus: Don't
plant directly in the ground. Anchor them to a piece of wood or rock with some fishing line till they take hold. Java fern my be planted in a shaded area.
Floating plants: Avoid trimming the root shoots. Can be planted with the use of weights. You can skip the weights if you have a good pair of planting claws which will allow you to set them nice and deep into the bottom material.
And always make sure to leave enough room around the roots for future growth -when planting near the back and edges leave some space in between the plant and the glass- this way growth won't be stifled and getting a scrubber in there for algae scrapping won't be more of a chore than it already is.