Red lentils dissolve to mush, so I would throw a handful per 250g meat into the slow cooker straight from the bag. They will absorb some water from the sauce, so make it a bit more liquid than usual. This works fine for bolognaise, but for something like chilli or a cottage pie filling I prefer green lentils.
Green lentils retain more of a texture and tbh it is difficult to say what is mince and what is lentil when you eat the finished product from a slow cooker. I do 50/50 with the meat, so it really does make a lot of portions for freezing.
I bring them to a boil with enough water to cover in a saucepan, let it simmer at a fairly rapid pace for 5 minutes, then turn off the heat and leave them to cool in the pan. This is usually overnight as I am getting ready to set the slow cooker off in the morning, but a couple of hours will do. If there is a lot of surplus water, drain that out before adding to the slow cooker, otherwise just tip in.