光源、つまり光の向きだな、それと陰影とかにも気をつけて、できるだけ正確に手元にあるクロッキー帳に書き取っていくだけ。

、、、。

Paying attention to the light source — that is, the direction of the light — plus the shading, you're just capturing it as accurately as possible in the croquis pad in your hand.

Literal

Light-source, in-other-words light [gen-の] direction [right-な], and shading [and-とか] [on-にも] pay-attention-[and-て], as-much-as-possible accurately at-hand [in-に] existing croquis pad [on-に] write-down-[goes-ていく] only is.

Informal art instruction. つまり ('in other words, that is') is a discourse marker for rephrasing. 陰影 ('shading, light and shadow') is a compound noun combining 陰 ('shade') + 影 ('shadow'). クロッキー帳 is a loan-native compound: クロッキー ('croquis,' from French for 'sketch') + 帳 ('notebook'). 書き取る ('to write down, to transcribe') is a compound verb. The ~ていく at the end adds a progressive 'continuing' nuance. The ~だけ at the very end compresses the instruction to 'it's just doing this.'