I have done the Consecration to Mary =) the first thing I’d recommend doing is reading St Louis de Montfort’s book “True Devotion to Mary”, that would answer many questions that you might have =) it’s a great book.
For me, when I started reading the book (actually I read the “Secret of Mary” first) I couldn’t understand it, and it seemed like idolatry to me… I asked St Louis de Montfort for prayers and that evening when I was praying the Hail Mary, I suddenly ‘got’ it and it finally made sense to me.
We could consecrate ourselves directly to Jesus, but if we do this consecration, then we’re giving ourselves to our Blessed Mother and asking her to give us to Jesus more perfectly. So the end result is the same, we are consecrated to Jesus, - but more perfectly because it’s through Mary. St Louis de Montfort gives an example of - giving a present to a king… if you give it through the queen, she can make it more beautiful.
In addition, it’s a great way of being under Our Lady’s protection and she prays for you and leads you further in holiness
Its effectiveness depends on how much you make use of it… St Louis de Montfort is very specific about this, he warns us that if we know this “secret of Mary”, it would be important to make use of it.
The day I consecrated myself, I was filled with many doubts and had some spiritual attack I think. But when I went to Mass and Communion, I felt a lot of peace and like God would like me to say the prayer, so I did. I had a lot of peace after and really felt closer to Mary, and like I truly belong to her now. I was glad I did the Consecration

later on, sadly I kind of fell away from practicing it well - through forgetting it, etc, - but I still renewed the consecration as you should do every year on the same day. (you choose Mary’s feast day… and there’s a 33 day preparation period). Now I’m looking to read the book again and renew my Consecration once more

and really practice it better from now on. There are certain prayers you can do, like the Little Crown of Mary, that help to practice it daily. But mostly the devotion is interior rather than exterior. St Louis describes this in detail in his book. (the book is available online or you could buy it, - personally I like to have the book in front of me in print

).
There was a bit of a discernment process at first, - I prayed about it but ultimately God gave me a desire to belong to Mary in this way and that’s why I did it. The preparation is intense and it’s best to maybe do the prayers in the morning or during the day and think about the readings throughout the day. A big aspect of this is praying to the Holy Spirit for light on the devotion.
You live out the devotion on a daily basis by doing everything through Mary, basically making her a part of everything so she can guide you and lead you to holiness. There are prayers that are recommended in the book to say, and although they are optional, it’s very good to say them daily. Don’t fear of not meeting the commitment, because as long as you remember to do it, you will. The Consecration would always be there unless you willfully renounce it (which would not be a good thing to do, but I can’t imagine anyone truly renouncing this Consecration if they love Jesus and Mary). It’s kind of like the morning offering: you do it and then it’s effective for the whole day… only this is for your whole life and eternity.

it’s very safe, there is nothing scary about belonging to Mary. She really takes care of you. And it’s not idolatry if you consecrate yourself to her in order to belong more fully to Jesus: as long as He is the end of your devotion. St Louis says this too. As for it being a vow, it’s not a formal vow like the religious make yet I think it has the character of a vow. There’s a responsibility in the same way as we have responsibility to make use of God’s graces… but it’s really about the effort, as long as you try to live it out, God and Mary would teach you the rest. The only problem is if a person forgets about it or is too lazy to remember it. The prayers and external practices are optional, though very helpful. Like I said it’s mostly an interior devotion; that is the essential part.
Regarding your last question… “What if one has prayed for increased devotion to Mary and has given intellectual assent to it but it is not yet firm in his heart? Is devotion to Mary a prerequisite or a result? Or both?” that’s a good question, I’d say that if you WANT to have greater devotion to Mary, that is already a grace from the Holy Spirit and this consecration can help you with that. The preparation process can also really help, much can happen in 33 days of prayer

if by the end of the preparation you really don’t feel ready, you can do it again for another feast day. But - based on my experience and the experience of some others I know, the devil might really try to stop you from consecrating yourself by suggesting various doubts, fears, etc, - so I think as long as you know that you can say the words and mean them, - don’t be afraid and do it
On the day of the consecration, you go to Confession (if you can, but a day or two before is fine too), Communion, and then kneel before the Tabernacle and say the prayer. You can then do something special for Our Blessed Mother like light a candle, give an offering to the poor, etc.
Hope that helps!
God bless! =)