Did you know diacritics can be easy to use on your mobile device?
On the standard Google Keyboard, Apple Keyboard, and some others, simply touch and hold your finger on the English version of the letter or symbol to pop up a dialogue with diacritic options including ‘okina!
Remember that ‘okina is not an apostrophe; it curves toward the word with the ball at the bottom (like the shape of a "6"). Touch and hold down the apostrophe key to find it!
Try it for āēīōū. Just touch and hold down the English letter until the different language marks show up, then slide your finger to the one you want to use! There are also options for other language marks using this method.
If the method doesn't work for you, you may need to install "Google Keyboard" or find a Hawaiian keyboard for your device.