The mental prayer 'norm' is 2nd on the list (I am doing the norms in the order with which they are read out in the weekly circle).
The Compendium of the Catechism of the Catholic Church no. 534 says
"Prayer is the raising of one's mind and heart to God....."
Of all the norms - this I find the hardest of all because it demands a great deal of mental gymnastics - and requires one to think. But it is absolutely necessary to pray daily - and as Pope Francis said recently - if you don't pray - you pray to the devil! How many times has this Holy Father mentioned the devil? - I have lost count! He did what appeared to be an impromptu exorcism on Pentecost Sunday - on a man with 4 evil spirits. Well he is the Pope after all! Our Lord gave his apostles power to do exorcisms and every diocese in the world has to have an exorcist.
But what did he mean though - that if we don't pray we pray to the devil? Because he likened the devil to worldly possessions and immoral behaviour. I think what he meant is that - if we don't pray to God, we love the world and everything it has to offer - which can lead us astray and into Satan's arms - sins of the flesh and all that!
The celibate members in Opus Dei (numeraries) and clergy are required to spend 1 hour in mental prayer daily - divided into 2 periods (ideally morning and afternoon).
The married members are supposed to do a minimum of half an hour daily - divided into 2 sessions. Flexibility is allowed though and sometimes I have to do half an hour in one go. If I know what's happening in my day - I can work out when to fit the mental prayer in. During school term-time it isn't a problem, because I can do some mental prayer before the mass begins. During school holidays I may only manage to do the prayer in the evening when my boy has gone to bed. Sometimes I do 10 minutes in the morning and 20 minutes in the afternoon. The goal for supernumeraries is to work up to the full hour, time and circumstances permitting.
What does one pray about? Well anything and everything! - God is there to listen to all your worries and concerns. You can pray about the Church, the sick, the poor, exams to sit, wayward children, lapsed Catholics, scandals to stop - God wants to know everything about you. You can also thank God for any good things that have happened and you can pray on behalf of others. Think of it as having a friendly conversation with our Creator. If you are angry at something or someone - tell Him! Sometimes you want to know if something you are doing is pleasing to God and you may want some concrete 'proof'. It may take a few days to get some 'sign' so you need to discern these things. That 'sign' can come through a person or an event perhaps. Discernment really is the key here.
In Opus Dei we are encouraged to take the day's Gospel to our prayer. A daily missal will help or any number of apps like "Evangelizo" have the daily mass readings. In the Gospel Our Lord speaks to us, although I must confess most of the time I don't know what Our Lord is saying to me when I read the Gospel. I think this is where Lectio Divina comes in - not that I have really tried that approach.
The other 'aids' to mental prayer are a series of 7 small books with a meditation for every day of the year. These are called "In Conversation with God" by Francis Fernandez. They are not supposed to be read like a novel - one is supposed to read them slowly and ponder the points. Other aids are 3 little books by St Josemaria - "The Way", "The Furrow" and "The Forge". These have 1000s of bullet points which act as 'triggers' to the brain - to get it praying!
So, there is no excuse NOT to pray. People say they have no time - but you can bet your bottom dollar they have time to watch some series on TV or watch sport. Everyone has time to pray.
What about praying while doing a physical activity? Well this is fine for say - walking the dog. But doing the ironing? You may end up burning the clothes! Ironing requires a certain amount of concentration so not a good idea to pray at the same time. Walking the dog - in a quiet field does not require too much concentration (unless there are sheep nearby and you have a terrier!).
What have you got to lose?