l am makeing a rogue subclass, and l want them to have magic. Any advice is welcome.

how will this work? l plan on makeing them a half caster (like ranger), so will the ranger spell slot progression be used? 

No. The only thing you can give them is the 1/3 caster progression like Arcane Trickster uses.

But under "spell prepare type", my only options are "full level" and "half level". How does that work to only allow the arcane trickster levels?

Those only work for base classes which we cannot do. You have to leave that blank.

Ah. That sucks.