Thomas Röhler (2016 Oly Champ, 2018 Euro Champ) put together a good series of How-To javelin videos., which is a useful resource for athletes and coaches. Via Paul McKee Athletics Ireland.
Episode 1 – Basic Equipment: https://www.youtube.com/watch?v=ryl6lFi4_Bw
Episode 2 – Warm-up: https://www.youtube.com/watch?v=3yN00vc82b8
Episode 3 – The Grip: https://www.youtube.com/watch?v=V3oW0XHSPvw
Episode 4 – Easy Exercises: https://www.youtube.com/watch?v=Ino8F3Df_2Q
Episode 5 – The Standing Throw: https://www.youtube.com/watch?v=i_Lx-6R5ICo