Usually the Yidam is chosen by the lama for the student and depending on the students' needs, the lama might prescribe a certain way of visualizing that suits the students' needs. If you do not have a Lama to assign you a Yidam, you can do the sadhanas that are meant for the uninitiated. At this day and age, many Buddhist bookshops will indicate if the sadhana or text requires an initiation before the practice can be done. It is best to stay away from practices that require initiations if you have not acquired any. The sadhana of the Yidam itself would usually indicate on how you should visualize the deity. This is stated in the invocation part. Take for example, as found in the Dorje Shugden sadhana:
HUM
I have the clarity of the Yidam.
Before me in the centre of red and black fire and wind,
visualize being surrounded by flames and a wind that are red and black in color, like what you see on flaming yidams.
On a lotus and sun, trampling demons and obstructors,
Lotus and sun here means the lotus base that the deity sits on. refer to the above as you can see the moon cushion on the lotus.Is a terrifying lion, which is powerful and alert.
Visualize the snow lion.Upon this sits the great king Dorje Shugden,
The supreme Heart Jewel of Dharma Protectors.
His body is clothed in the garments of a monk,
And on his head he wears a round, yellow hat.
His hands hold a sword and a heart of compassion.
To his followers he shows an expression of delight,
But to subdue demons and obstructors he displays a wrathful manner.
He is surrounded by a vast, assembled retinue
Such as his attendant Kache Marpo and so forth.
Light rays from my heart
Instantly invite the wisdom beings from the sphere of nature
And from all the palaces where they abide.
They become inseperable from the commitment beings.
From the sadhana, you can see that it is exactly like how the image is. An easy way is to just visualize the yidam as according to the picture, without referring to the picture, and as detailed as possible.
