Hey c:
If you draw it on an art program out of Chicken Smoothie, you’ll have to take a screenshot or save the drawing as png or jpg. Then you’d upload it onto a website, maybe sta.sh or imgur.com, deviantart.com, those three are the ones I personally use ^^ then you’d copy the images link, and send it over to your client! Pretty simple.
For Oekaki, all you’d have to do really is upload it onto Chicken Smoothie and then either link the artwork to them, or tell them that you have finished and they just have to check your gallery to find the artwork !!
If it’s traditional artwork (paper, pencils, etc) then the same would apply to the very top, take a photo and then upload it onto a website, then link it to them c: hope I managed to help! Feel free to send over a PM if you’re still confused, I’m always willing to help ^^