How to meditate upon LORD?

Arjuna asks this question in six verses 10:12 - 10:18

The most important of the six verses is:
कथं विद्यामहं योगिंस्त्वां सदा परिचिन्तयन्।
केषु केषु च भावेषु चिन्त्योऽसि भगवन्मया।।10.17।।

10.17 O Yogi, katham, how; aham vidyam, shall I know tvam, You; sada pari-cintayan, by remaining ever-engaged in meditation? Ca, and; kesu kesu bhavesu, through what objects; bhagvan, O Lord; cintah asi, are You to be meditated on; maya, by me?

Lord Answers this question in several verses from 10:19 till 10:42 (end of Tenth Chapter)

There are primarily two methods of meditation of Lord:
अहमात्मा गुडाकेश सर्वभूताशयस्थितः।
अहमादिश्च मध्यं च भूतानामन्त एव च।।10.20।।
10.20 अहम् I, आत्मा the Self, गुडाकेश O Gudakesa, सर्वभूताशयस्थितः seated in the hearts of all beings, अहम् I, आदिः the beginning, च and, मध्यम् the middle, च and, भूतानाम् of (all) beings, अन्तः the end, एव even, च and.

1. As all-pervading atma (the very subject). If one can't do that then
2. As most glorious object in a specific class of objects.

(10:21)adityaanaam aham vishnu .... jnanam jnanvataamaham (10:38) lord gives different objects that can be used for the most glorious object for meditation.

in 10:39 Lord states, "I am the primeval seed from which all creation has come into existence. I am the seed of everything. I am the Self of everything. Nothing can exist without Me. Everything is of My nature. I am the essence of everything. Without Me all things would be mere void. I am the soul of everything."

again in 10:41
10.41 यत् यत् whatever, विभूतिमत् glorious, सत्त्वम् being, श्रीमत् prosperous, ऊर्जितम् powerful, एव also, वा or, तत् तत् that, एव only, अवगच्छ know, त्वम् thou, मम My, तेजोंऽशसंभवम् a manifestation of a part of My splendour.

and finally concludes:
अथवा बहुनैतेन किं ज्ञातेन तवार्जुन।
विष्टभ्याहमिदं कृत्स्नमेकांशेन स्थितो जगत्।।10.42।।
विष्टभ्य supporting, अहम् I, इदम् this, कृत्स्नम् all, एकांशेन by one part, स्थितः exist, जगत् the world.

"Having established or pervaded this whole world with one fragment of Myself, I remain."

om tat sat

My Notes
Note 1: The Universe is called a foot of His by virtue of His having the limiting adjunct of being its efficient and material cause. (figuratively!) A Form constituted by the whole of creation has been presented in this chapter for meditation. In absolute reality, HE has no limiting adjunct. The limiting adjunct belongs to the one who is meditating on him.... called ignorance.

Note 2: Second method is no inferior to the first one. Both are equally effective, but one should choose most effulgent object in the chosen group of objects for the meditation.