In general, some guidelines you will want to follow:
Prefer rounded corners on traces over 45s/90s
Where pads go to traces, you will want the delineation to be gradual instead of sudden (e.g. don't take a 20 mil wide trace and have a 5 mil wide line exit directly from it). You do this to avoid stress fractures when the traces need to bend. An approach not unlike how you'd teardrop a via should generally be OK.
If you need to solder the flex to a rigid PCB, you will want to provide plated through vias in the area where you're going to be soldering. Ideally you'd have matching pads on both sides of the flex, but if that doesn't work for whatever reason, the vias will give you a reasonable chance at soldering without a hotbar bonder.
The price roughly doubles per additional layer, so if you can get it done in a single layer...
If the flex does need to be, keep to about 3x flex thickness as your minimum bend radius. Also, prefer bends that are inward vs. outward. If you bend outwards with a sharp corner, you risk getting trace breakage that doesn't happen immediately. It will happen after your flex has been "in the field" for a few months. I've personally made that mistake.
For small qty, you will probably get laser cut flex. Watch out for carbon marks/charring around the cut area. I'd leave a good margin of clearance from the cutout. Die trimmed flex will not have this problem, but it's not really economical in smaller qty. Steel rule dies can be made cheap, but the quality and consistency of the cut is not generally good.
If you want to do assembly on the flex, consider either partially rigid flex, or stiffeners in the areas where components need to mount. If that's not an option, SMT assembly is still an option, it just generally requires the use of a vacuum table and special handling in the reflow oven.
I think that's about all I can think for now.
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