At times, in the morning, as soon as I wake up, I try to incline my minds towards thoughts of metta, i.e, do some reflections to connect the heart to metta (instead of feeding the grumpy mind that is lazy to get up...). I relax the body, being aware of the body still lying in bed
:):) - pleasant feeling...
Then, I try to recollect, remember how is the mind when there is metta (when I did experience it previously): the mind is open, spacious. There is a quality of giving - offering sincere thoughts of loving-kindness, in an unconditional way. Wishing myself and others to be happy freely, without any condition, grasping or holding. Just offering thoughts of tender care and love... no matter what... So I try to direct the mind towards those thoughts and aspirations and nourish the intentions : may whatever I think, say and do today be motivated by metta, the expression of metta.
In the morning sitting, I used to do a few minutes metta before vipassana as one of the 4 guardian meditations. But at times, whenever needed, metta meditation could last more, 15, 30 minutes or even more. Presently, the metta practice comes naturally after the vipassana. After vipassana, the mind is clearer, purer. When there are fewer hindrances in the mind, there is more room for metta to flow naturally and freely.
When I did metta before as a guardian meditation, I would also shift to vipassana while practising metta if the mind finds out that there are hindrances. For example, once I was radiating metta to an 'ennemy'. As her face propped up in my mind and I was trying to send her metta (may she be well and happy...), feeling of aversion arose. So I noted them. They disappear, and thoughts became clearer. And there was feeling of fear. So I let myself be aware of the thoughts and observe the feeling of fear. After a while, it disappeared. I came back to the metta practise.
Then throughout the day, I try to send metta whenever I remember to do it : while queuing at the market (I see all the people who have the good kamma to be able to buy all these things, and wish they can buy whatever they need, that they may be well and happy ---- instead of nourishing thoughts like 'oh my gosh, exhausted by all this crowd!!!)