I have created a tutorial for this.
the hardest part of this for me was aligning the cross hair and the bow aiming direction of the character. For this I had to use rotate the character from a bone using Transform modify bone node in animation blueprint on top of the animations. Here you can see the process I have followed. Unreal Bow and Arrow - Camera Aiming down and Crosshair - UE4 Tutorials #173 - YouTube
