Long toss comes down to two phases. It is not mindlessly throwing for long amounts of time. In the beginning you should start small and build up volume.
This document has a series of videos that are easy to implement and based on sound principles of human movement. Coaches, parents and kids can all learn some good drills and movements to develop into solid throwing pitchers.
From the Florida Baseball Ranch, here is a quick, easy and effective way players can regain range of motion after throwing.
There are a lot of misconceptions regarding long toss. This article is a great starting point.
Age level guidelines published by the Major League Baseball initiative to protect young arms. MYAS rules provide pitch count + innings pitched guidelines, and ROYBA coaches all follow these guidelines.
Published by Major League Baseball, the article provides a lot of risk factors for youth throwing. ROYBA strongly encourages all coaches to adhere to these guidelines.