grammar - ～ておく or ～とく for preparation (conjugation and nuance)

A few quick questions regarding ～ておく and the casual form ～とく

Firstly, when changing from ～ておく to the more casual ～とく　I'm assuming the verb is first conjugated to the ～て　form then the ～て is dropped and replaced with とく and it can then be conjugated following the godan conjugation pattern as in the examples below.

I.e.

勉強して　→　勉強しておく　→　勉強しとく

～とく conjugates as godan such that 勉強しときます and　勉強しときました. In the case of verbs that have a ～で ending do they conjugate as どく？

I.e.

読んで　→　読んでおく　→　読んどく

～どく　also conjugates as godan such that 読んどきます and 読んどきました

Is my understanding correct here?

Also, are there any nuances that make it outright different from ～つもりです in terms of preparing for something?　Is it simply that one of the sentences below feels more natural than the other?

For example

明日試験があるので今晩勉強しておきます
vs
明日試験があるので今晩勉強するつもりです

Thanks

Answer

Yes, all your assumptions about about the conjugations are correct.

And far as comparing it to つもり, つもり simply means "intention (to do something)". It doesn't directly have anything to do with preparation or doing something beforehand. That it carries this mean in your example is incidental. With your 勉強しておく sentence, the preparation is explicit; with the 勉強するつもり sentence, the preparation is implicit. But in general, using つもり is not for preparatory situations.

• 昨晩雪がたくさん降ったので、週末にスキーに行くつもりです　→　It snowed a lot last night, so I intend to go skiing this weekend.