Smashan Kali is mentioned in Damar and Yogini tantra, and appears in Kamakala Khand of Mahakala Samhita too. Her iconography is as follows as mentioned in Yogini tantra -
She is dark in complexion, and her hair disheveled. Her eyes are bloodshot and Her teeth is terrifying. Her body is gaunt and unclothed. She sports a long lolling tongue. In her left arm she carries a Kapala (bowl made of skullcap) containing blood, wine and meat. In her right hand she holds a decapitated head, from which blood is flowing. She is chewing on human and animal flesh constantly and rides in a cremation ground.
For Guhya Kali a complete khand is dedicated to her. Guhya Kali khand of Mahakala Samhita states her iconography as follows-
Guhyakali, Mahakala says, has forms with 100, 60, 36, 30, 20, 10, five, three, two and one faces. Different mantras correspond to these different forms, which he then reveals, using the usual codes for the different letters of the Sanskrit alphabet employed in other tantras.
Mahakala commences the discourse if
Guhyakali in simhasana, on her lion. She gives the dikpala or guardians of the directions to the five great corpses (Panchpreta) of Shiva on which she sits.
Bhairava is the sixth pitha, described as black complexioned with four arms. Bhairava appears terrifying and he is the cause of fear. He is five faced, each with three eyes. There is a khatvanga, staf and scissors in his hands. He also holds a skull and damaru. Garland of skulls adorn the fanged Bhairava. He is on an eight petalled lotus. Four petals of the major directions symbolize dharma, jnana, vairagya and aishvarya. Varigya is dispassion and aishvarya means dominion. Shiva, above Bhairava is clothed in tiger skin. He is two-armed and holds a skull-staff and a trishula.
Mahakala then describes the twenty-seven eyed, ten-faced form of Guhyakali and her animal aspects represented by each of her faces. Dvipika: a leopard or possibly a panther; Keshari: a white lion; Pheru: black jackal; Vanara: red moneky; Riksha: purple bear; Nara: cochineal colored woman; tawny Garuda; Makara: turmeric color crocodile; Gaja: gold color elephant; Haya: dark or shyama color horse.
Jagadambika, Mother of the Universe has a terrible lolling blood dripping tongue. Her laughter is sharp and shrieking. Both her garlands and earrings are skulls. She has fifty-four arms. In Her right hands she has rosary, scissors, goad, a rock, a danda, a jeweled pot, a trishula, five and arrows, a discuss, a bamboo stave, a fire hearth, a barbed hook, a man’s skeleton, young corpse and threatening mudra. She holds a bhindipala- a sling and shataghni- club studded with metal nails.
Guhyakali dwells within the centre of eight cremation grounds. These shmashans are: Mahaghora, Kaladanda, Jvalakula, Chandapasha, Kapalika, Dhumakula, Bhimangara, and Bhutanatha.
Her worship honours the Vetalas or vampires, eight tridents, vajras, jackals and corpses, Bhairavas, dakinis, Chamundas, Kshetrapalas, Ganapatis and other denizens of the cremation ground.
As in what the Tantra background/stories are for these forms of Kali
Not all forms have anecdotes or itihas related to them. Yet Guhya Kali is mentioned in Devi Mahabhagwat when Durga rages war against Durgamasur. Also, smashan Kali has no itihas liked to her.
what the sadhana methods/techniques are, if possible.
Actually tantra sadhanas can never be learnt online, and even if they are, then to practise thoe diksha is required. Thus let's wait till Bhagwati graces us with Guru.