Nirguna upasana is not about "not accepting" murti puja. You are thinking about it in negative way. Nirguna means without qualities, without form. It is worship of formless aspect of God. It is not about rejecting various forms of God. It is just different path for people whose personality is not attracted to any of the forms of Lord. It is done by meditating on immutable, indefinable, changeless, all pervading, unmanifest aspect of God, by practicing control over one's sense organs, by being equanimous and unmoved by ups and downs of life and by doing selfless service of others.
12.3 Those, however, who meditate in every way on the Immutable, the Indefinable, the Unmanifest, which is all-pervading, incomprehensible, change-less, immovable and constant.-
12.4 By fully controlling all the organs and always being even-minded, they, engaged in the welfare of all beings, attain Me alone.
Edit: Control of sense organs practically would mean practice of yama-niyamas given in patanjali yoga sutras. http://www.swamij.com/yoga-sutras-23034.htm
Meditation on nirguna would involve meditating on Om by mental or verbal japa of Om. Or japa of aham brahmasmi. http://www.swamij.com/om-mantra.htm
To understand difference in attitude of saguna vs nirguna upasaka may be this article will help. http://www.sivanandaonline.org/public_html/?cmd=displaysection§ion_id=1442
To understand more about philosophy of nirguna brahman, though not strictly necessary for practice of nirguna upasana, see this video. https://www.reddit.com/r/hinduism/comments/6kevj6/definition_of_god_based_on_taittiriya_upanishad/