HOW TO MAKE BONE BROTH
Bones...
Bones: Ask your butcher or supplier to make sure they're small enough to fit in your pan.. You can decide the mix, beef and chicken work well together. Lamb and beef bones are more fatty and marrow bones are less gelatinous and more flavoursome and fatty. Stock bones with cartilage have more gelatine which will cause the broth to gel more which is what many strive for and if you have the choice a little meat left on the bone never goes amiss.
Have a play around with a mix and work out what you like. For health and safety reasons I'd suggest using a slow cooker. You can buy slow cookers quite easily these days with plenty available online.
The cook...
In my experience if you want to use the hob of your oven, leaving it on the very lowest heat setting overnight with enough water in the pan will be fine, but do this at your own risk.
If you've never made it before it might be worth starting your first batch in the morning and ending the cook at night after 12-14 hours so you can gauge the temperature of your lowest setting on your oven and understand how safe it might be to leave it overnight next time. Every oven varies. However if you want to cook it overnight, I'd recommend starting it just before bed so the water level is at its highest and risk of the pan drying is much lower.