Abou how to record mi Paintball Action

Hi Everybody. I'm new at this kind of things.
But i enjoy a lot playing with my team Paintball\airsoft matches. And at the moment i managed to put my android used samsung on top of the replica but my problems are :

• how to get a red dot on the image
  . and if possible adjust it , like the aimpoint we can buy for games.

I will try to have some pictures so you can understand my "thing"
Thanks for all ideas
best regards
Vin