Do:
Pray that their spiritual eyes and ears of understanding would be opened.
If you don't, avoid them. They have more twists than a pretzel and they will be able to back up all their beliefs. Those who know the real thing are best able to determine a counterfeit. Listen attentively. This shows you care about them.
Which terms should you define? God: The father, Jesus, The Holy Spirit, The Trinity, and salvation. Story about Doctor who does not take meaning of words seriously.
Remember they believe they know more than you. Ask 'please explain'. This puzzles me. This appears to be a contradiction. Jesus continually asked questions.
Remember that their Bible is doctored. Ask them if they would please read this verse and what does this mean to them? Treat them as people, not as cultists. Even though they are part of a cult, remember that Jesus died for them also.
Go over a point 10 times if needed. This is easy to say but hard to do, especially if they appear as 'know it alls'.
If you don't know answer, tell them you will get an answer for them at a later time
Say something like we all want the truth, don't we? We can win the point but loose the cultist. Others: Always stick to one point until finished. They will try to jump around.
Don't:
Like liars, deceivers, Antichrists, etc. Most are sincerely deceived. Watch your tone and non-verbal communication. This can be the most difficult part.
This may be communicated non-verbally.
Give some examples of Christians witness to JW's. Slamming door in their face only reinforces their deception (after all they are suffering for their faith).
They may feel trapped. Other don'ts: Don't dodge their questions. Try to answer each one.