A policy is what your delegation wants the committee to do, and why. It is the difference between knowing about a topic and being able to negotiate on it.
Write your policy down before the conference. During lobbying you will be asked what you want within the first few minutes. You will not have time to work it out then.
Your stance. One sentence. What your delegation thinks should happen. If it takes a paragraph, it is not a stance yet.
What you want. The two or three specific actions you will push for. Name the body that would carry each one out and say where the money or the people come from.
What you will accept. The weaker version you can live with. Almost nothing survives lobbying unchanged, so decide in advance which parts you will trade.
What you will refuse. The clauses your country would vote against however the rest of the resolution reads. This is the line you defend. Knowing it stops you agreeing to something in the room that your country would never accept.
Before an action goes into your policy, put it through three checks.
Is it within the mandate? A General Assembly cannot impose sanctions or deploy troops. If your action needs either, it belongs somewhere else.
Would your country actually support it? Not whether it is a good idea. Whether that government would sign it.
Can it be carried out? Say who does it, with what, and by when. An action nobody can perform is a sentence, not a policy.
An action outside your committee's mandate can usually be saved by rewording it during lobbying. Change "Demands sanctions" to "Strongly urges the Security Council to demand sanctions". The intent survives and the clause becomes something your committee is allowed to say. See Motion to refer to another forum.
List the delegations that share your position and the delegations that oppose it. Note what each of them wants that you could support, and what you want that they might support. That list is your lobbying plan.
In MEDIMUN, lobbying starts online before the conference. It continues on day one, through the call feature in MediBook. Alliances are declared in MediBook. Have your policy written before lobbying begins. See The lobbying process.
If your country's position is unpopular in the room. Represent it anyway. Say what your delegation cannot accept, and say what it could accept instead. A delegation that negotiates from an unpopular position gets more into the final text than one that abandons it.
If your policy and your own view are opposites. Argue the policy. Your job is to represent the country accurately. Nobody in the room will assume the speech is your own opinion.
If you represent a permanent member. Your policy has to include when you would use the veto and when you would not. It is used very rarely in reality, and a delegate who threatens it often is not representing their country accurately. See The veto.
Non-permanent members of the Security Council are expected to reflect real alliances and real economic ties. An organised bloc formed against the permanent members is discouraged and should draw a warning from the chair. See P5 and non-permanent members.
If you are unsure whether an action is too strong. Ask what your country's own government would do if the resolution were binding on it tomorrow. If the answer is that it would object, soften the wording now rather than in front of the house.
Once the policy is settled, it goes into your position paper and your opening speech. See Writing a position paper and Opening speeches.