Different lineages / teachers would advice on this differently. My answer is based on what I picked from different teachers.
Before doing metha best to to do a bit of breath meditation and insight meditation. Come to the stage that you have Piti or Passaddhi or at least you feel comfortable. Then wish that other share your peace and happiness. Keep wishing your self and others to be well and happy. Ideally keeping your attention around your mouth. Lips touching or above the upper lip and base of your nose. Also try to bring out a mild smile (this generally relaxes tension) as this will influence your mood. Also in the process if possible try to calm the fabrications, i.e., realise the speed and length of your breath from start to end.
This should not a mere recitation of words. You should wish from the bottom of your heart. In the process uproot your defilements to make your mind pure.
One of the objective of Metta Bhavana is the reduce the defilements based on aversion, dissolving ego as well as the hindrance of ill will, especially towards other beings. So you should be mindful when doing it that aversion does not arrive and also you are not in any discomfort as this also sometimes lead to aversion and tension (which intern creates new unwholesome fabrication and conditioning). Doing this after breath meditation and awareness meditation can help the cause in identifying 1) defilements arising based on aversion 2) any hidres arising due to enmity or any pre conceived notion of the person. Always be aware of the metal factors arising during the meditation and if they are unwholesome try to tame them by being aware that they have arisen and the sensations they bring up and then the dissolution of it.
Also when you think of some one keep a note of how thoughts proliferate and what feeling this brings up. You think about a friend you might then remember some other incident with them. All this creates feeling and sensations when you react to them. Try to be equanimus. Similarly when you think about a foe or even an attractive person. Also then you think about animals or other life forms, you might experience emotion, feelings and sensations. Suddenly you might remember a rat who ate your favorite piece of furniture. Or the fear of a snake when you think of this type of animals, or the disease they spread if it is a cockroach or rat. Then these thoughts arise you mind make a contact followed by reaction and sensation. Keep focused on this element. The sensation this causes trying the be equanimous about the sensation and realising the sensations have the 3 marks of existence. Also notice how your are reacting or sensation that are created with perception of an entity / being. Look at the feeling when you recall incident or people who attacked your ego or what ever you call me, mine, etc. and realise how ego is hurting you. Also realise how your react when remeber someone praises you or has been benevolent to you. Also look at how perceptions about an entity / being also contributing to stress in the process. Look at mental image -> recognition -> reaction -> sensation -> craving -> clinging ... Try to see the dependent origination, factors of enlightenment and 4 noble truths at work even through Metta meditation. When practiced in the right way Metta can take you far.
Meditation based on the Four Sublime States help expand upon the type of defilements which to reduce through these meditations.
If you feel any discomfort take this into awareness while radiating Metta. Keep your attention on these parts which hurt. Also be mindful of perspiration while doing it.
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