Dr. Scott Oliphint has a short lecture (about 20 minutes or so) titled "Apologetics and the Doctrine of Scripture," in which he explains Reformed Apologetics. In Oliphint's view, Reformed (he likes the label "Covenantal") Apologetics are simply the apologetic implications of Reformed theology. In other words, theology informs apologetics - the two are not entirely separate endeavors. But what are the apologetic implications of Reformed theology? The primary implications relate to an epistemology of divine condescension.
God is known by Revelation (both general revelation and special revelation). Creation is related to God (either positively or negatively). Thus, a right understanding of Creation as a whole requires a right understanding of God, since the two are related. A right understanding of the relationship between Creation and God is obtained by Revelation. Since the Revelation comes from God, it is necessarily true. The clearest form of Revelation is verbal revelation, and indeed it is verbal revelation that serves as the basis for understanding general revelation. Scripture is the verbal revelation we currently have.
Therefore, in apologetics, as in evangelism, we present the truth of Scripture as against the claims of rival systems. Since the rival systems are necessarily false, the apologist's job is to help the unbeliever understand that the rival system is false, as well as to help the unbeliever understand that the Scripture is true.
In other words, the fundamental positive assertions of Reformed apologetics are (1) God has spoken and (2) it is our (humans') privilege and duty to do what God has commanded us.
William Edgar explains (here) a little different aspect of Reformed apologetics. He explains a general approach:
1) We acknowledge that unbelievers hold much true knowledge (knowledge that is objectively true).
2) We explain to the unbelievers that some of their beliefs are inconsistent with this true knowledge.
3) We invite the unbeliever to learn/believe/experience the truth of Scripture.
Of course, the steps aren't necessarily in that order, but rather are a general outline of the aspects of conversation we use in evangelism and apologetics. The steps are premised on our position that we know the truth because God has revealed it to us in Scripture.
I would add that this should govern our apologetic priorities, our apologetic targets, and our apologetic balance. For example, the authority of Scripture (as against that of the Roman magisterium, or that of the alleged prophets Mohammed or Joseph Smith) is a high apologetic priority, whereas pointing out the clash between Islam or traditional Roman Catholicism and post-modern ethics is something that would be a very low priority. That's partly because post-modern ethics are not especially faithful to Biblical ethics (even though they haven't entirely eradicated the light of nature). On apologetic balance, the point is that while there is a time for negative, internal critiques of other systems, this negative material must be supplemented by a positive presentation of the revealed truth of God in Scripture -- not necessarily in every presentation, but certainly not rarely.