The flash umbrella adapter connects the flash and any umbrella to the light stand (or clamp). So you should buy a quality adapter up front, as it is cheaper than a new flash if the bracket fails. Look for the following features:
- Strong enough to hold the flash and any umbrella or light-box.
- Be adjustable.
- It must be able to securely fix the flash to the light-stand (or clamp) safely.
The umbrella slot needs to angled so the flash faces towards the centre of the umbrella. If the slot is flat, the flash fires off centre into the umbrella giving unbalanced light and there is nothing you can do to fix it. - A tear drop shaped umbrella slot (as opposed to round) minimises the risk of the umbrella spinning or the arm being bent when the knob is tightened.
- The cold shoe must not short circuit the flash, older style film day cold shoes can do this.
- The knob to tighten the cold shoe onto the flash should be big enough to tighten or loosen easily. Having a knob to tighten the cold shoe gives you 2 points of contact (the other being the normal way a flash is fixed to a camera hot shoe by a dial or lever). Avoid cold shoes with no knob to tighten onto the flash base. Strobframe make a good cold shoe.
- Forget about plastic adapters, they loosen and break too easily.
- Enough flexibility to fit to various stands and flashes.
Two adapters I know of meet these requirements the Manfrotto 026 and the Phottix Varos Pro.
Manfrotto 026 Lite-Tite Swivel Umbrella Adapter. It is mostly made of metal, strong and flexible in its design. You must also buy a cold shoe to attach the flash to the adapter. Full review here.
Phottix Varos Pro S Multi-Function Flash Shoe is made of alloy, it is strong, adjustable and comes with a cold shoe. Full review here.
Miscellous
If you are new to off camera flash, a visit to strobist.com is a must. Read through Lighting 101 a few times here.
Then proceed to Lighting 102 (Lighting 103 is being rolled out in 2017) here.
I would then strongly suggest you read the first version of Lighting 102 which is heavily focused on various projects, here.
Strobist Starter Pack, here.
Strobist (bigger) Gear Guide, here.
Then look at various Strobist assignments here.
Strobist Videos here.