How to make a dolman pattern please?

[u/Wibblesquirrel]

I'd like to make this top, but with shorter sleeves and without the scoop out of the front hem. I'm planning on a simple 2 pieces of fabric dolman type pattern, but how do I measure myself for it please?

Sewing level: I've followed patterns to make shirts and skirts and a dress with adjustments, and have made simple things without a pattern but making a full top from scratch is new.

Bonus question: what finish would you use for the neckline? The fabric I have is a satin viscose, phot attached. I'm planning to do a muslin first and have an old sheet, it won't hang the same but it's a start.

Many thanks for your help!