Custom Mob

Hello, im very new to data packs and i wanted to make a custom mob for a friend to use in our server, ive made the custom model and texture and ive looked at youtube for a few hours trying to figure out how to spawn it and now that i have the command to do that i have no clue how to actually add the model into the game if you could help it would be amazing below is as far as i have gotten i do also have an invisible mob with just the flint showing