Here are a couple good ones I know of:
There are two books I am aware of with specifics to Splunk App Developing:
(just for transparency - I wrote the Splunk Developer's Guide 😄 )
Take a look at those, read through dev.splunk.com about a bajillion times, and ask lots of specific questions. Try doing it once or twice. Might be easier to start with a Converted Dashboard (HTML) to see some of the under lying code.
Also - you can find me on IRC (#splunk on Efnet as alacer) , The Splunk Usergroup Slack (sign up at www.splunk402.com/chat) and here on answers, among other resources.