First we should look at what deep doctine actually is. It is often associated with things we don't know. But its true meaning would be better defined as things that are beyond that being delivered in Gospel Doctrine classes.
We have a standard doctrine that is being taught in Gospel Doctrine classes. But many recognise the importance of developing our knowledge beyond the milk. They will attempt to get the conversation in the GD class to advance beyond where it is. But this will fail because the reason it is where it is, is because the class, as a whole, don't wish to advance beyond where they are.
The same applies to priesthood classes. As I hear the questions given and the answers I feel like someone has put 5 cents in the slot and pushed the button for answer number 49. "Why should we do Home Teaching, brethren?" "That's right, because Christ said to." That's deep. I'm glad I was there for that information.
So many in the church cry out for something better than a reply by a ventriloquist's doll.
The problem on the other hand is that so many want to become doctrinal experts so that others will hold them in a place of honor. We end up with sites dedicated to speculation: Groups will discuss the subject all presenting their personal opinion. These will be discussed backward and forward with absolutely no real conclusion. Thus people are lucky to actually learn anything; which defies the point of such discussion. But they see themselves as some doctinal expert. They miss the simplistic answer because their heads are too much up in the clouds. They have gone well beyond the mark.
So what is the answer to learning deep doctrine?
First we must have the right purpose. We plan on being able to assist the endless intelligences that wait for some God to come along and give them these opportunities. Are you willing to help out? If so then you will need to know what you are doing. Then there are those things that if you search your heart honestly you will find you want to know. You needn't hide your head in the sand and pretend you can't see them anymore. You can come out and go to God with true purpose of heart. And if you do this sincerely you will begin to get those answers.
Getting the Holy Ghost as a constant companion doesn't just come because someone placed his hands on your head with authority and says the magic words. You have to invite him in by listening to his counsel and following it. In the process he will help you learn the answers to the questions that others just wander around asking but never have an answer to.