You will need to determine what type of button or buttons you want to use to activate the device. While it is possible to do switch control with a single switch, if the user is capable of pushing more than one button it will be much easier and faster. In single switch mode, iOS itself jumps from item to item and then you have to press a selection button to select that particular item. If you have 2 switches, one of them is used to manually advance the cursor to the next item while the other is used to actually select. Using three switches gives you the opportunity to scan forwards or backwards to the available options and not just continue to scan forward. The examples in this tutorial assume we are going to use three switches however we could use fewer or more depending on your needs and capabilities.
The type of switch that you use will depend upon the capabilities of your user. For people with plenty of strength but a lack of control such as users with cerebral palsy or those recovering from stroke you might want to consider this a large red arcade button.
This is a 60 mm diameter button but there is also a 100 mm version and they come in not only red but, green, yellow, blue or white. If you want something a little smaller you might consider this 30 mm arcade style button.
It also comes in a variety of colors.
Keep in mind that fingers, hands, elbows are not the only options available. The arcade style button could also be mounted on a wheelchair headrest to be pushed with your head. You might want to consider a foot switch such as this one.
If the intended user has limited range of motion and possibly limited strength such as those with muscular dystrophy or ALS then you will probably want a smaller button with a soft touch. A variety of tactile pushbuttons are available from Adafruit such as these…
Personally I have a type of muscular dystrophy known as Spinal Muscular Atrophy and I only have limited use of my right hand. I use micro switches similar to these
Note that you want a momentary contact button or switch not a toggle switch, slide switch, or push off/push on power switch.
Although it's beyond the scope of this tutorial, you might want to use an analog joystick of some kind. There are several available in Adafruit store. You may not want an actual physical switch at all. Set up a photosensor and LED as if you pass your hand or finger between them it would break the beam and cause a signal to be sent to your Arduino. You could mount an accelerometer on a lever so that simply moving the position of the lever detects the change in angle. Alternatively you might want to consider some sort of capacitive touch solution. Here are a variety of capacitive touch devices available from Adafruit. https://www.adafruit.com/categories/60
Also most Arduino compatible devices can be wired up to detect capacitive touch with nothing but a resistor and two input pins as described in this tutorial.
Although that tutorial talks about the flora, the capacitive touch principle will work with most Arduino compatible AVR processors such as ATMega328 and ATMega 32u4. If you want to learn more about how capacitive touch devices work, we recommend this video "From the Desk of Lady Ada" where she describes capacitive touch in detail.
Professionally built adaptive technology devices refer to the switch devices as "AT Switches" for "Adaptive Technology Switches". The industry standard for connecting such devices is a 1/8 inch or 3.5 mm mono plug and jack. If you want your device to be compatible with this industry standard, you should put a 3.5 mm jack in your device and a 3.5 mm plug on your switch. Here are some sources Digi-Key. Similar devices may be available at your local RadioShack or other suppliers.
Once you have selected the proper buttons, you need to consider where and how to mount them for the convenience of the user. If you have access to a 3D printer you may want to build a specialized enclosure for this project with the buttons mounted on the top. Here is a video from ATmakers.org that talks about how to mount commercially available AT switches on a standard camera mount. Use their solutions or use it to inspire you to create a mount of your own. https://youtu.be/Y-_8y-hHtM0
You may want to create a specialized piece to assist the user to hold onto micro switches in one hand. Here's a photo of a set of three micro switches in a 3D printed ring that helps me hold the buttons in my right hand where I can push the buttons with my thumb.
Each user will have different needs. You can use your maker skills to adapt and create something unique to the individual that may not be available in a commercially manufactured product.