How to Focus consciously? I have been trying to read while being conscious about myself or breathe but it seems impossible: I can think about only one thing at a time, and even if I do it "consciously" then I have to go through that thing over again.

So how do I practice the iOS Development (coding takes your entire attention), Which seems impossible to do consciously?

In other words, how do I focus while being aware of myself or breathing etc.
How to Focus consciously?


The Right Mindfulness is being equanimous towards the pleasant and unpleasant, which prevents craving and aversion from arising, while free from corruption of insight (free from Vipallasa, i.e., seeing the Three marks of existence), which prevents ignorance arising, within the 4 Frames of Reference. If unwholesome roots arise, deluded (with Vipallasa), hence creating fabrications  or based on external objects or conceptual objects hence not within the 4 frames of reference it is not right mindfulness.

If you are not equanimous and there is a pleasant sensation this leads to attachment and if there is an unpleasant sensation this leads to aversion and if neutral sensation, without insight into the 3 Universal Characteristics, this leads to ignorance. Hence, unwise attention is attention corrupted by Vipallasa and the experienced sensation is leading to the unwholesome roots. See commentary of Yoniso Manasikāra Sampadā Sutta &amp; Vipallasa Sutta by Piya Tan and Kīta,giri Sutta


  
    For, one who perceives impermanence, Meghiya, establishes the perception of non-self. One who perceives
    non-self eliminates the ‘I am’ conceit. He attains nirvana here and now.”
  


Meghiya Sutta

Similarly of the other opposites of Vipallasa (3 universal characteristics) can be understood through, impermanence, or seeing arising and passing.

So simply put it what needs to be done is:


stay with the sensation in the body
be equanimous - displaces craving and aversion
know what ever experience is impermanent, i.e., arising and passing - displaces ignorance


Also see this answer.

If you mind wanders away and you lose attention get in touch with the sensation than are prevalent in the body and mind and establish your attention on the 4 Foundations of Mindfulness.

Once you experience a gross (easy to penetrate) sensation try to experience its subtleness (had to penetrate) by strongly focussing on it and trying to divide and dissecting it.


  (coding takes your entire attention), Which seems impossible to do consciously?


Generally when doing something like this your attention will be lost. Thinking creates sensation in the head and chest regions. If you cannot figure something you the frustration means you hear rate increases and breathing becomes heavy. Similarly when exited. Sometimes thinking too much gives a headache. If your figure something out, or your code works, you will feel happy, which gives a pleasant sensation. It is these sensation you should be looking at. This will require extra practice, but what is essential is you are mindful when you do you daily work. The training on the cushion or chair is just a booster or pre training for this. 

Also additional information can be found in:


The Importance of Vedana and Sampajanna
The Four Sampajanna
Why Vedana and What is Vedana?