If your cars battery is completely flat and you can't get the bonnet or boot open or the gearbox out of park to push it out of the garage to jump start, you will need to power the car up with jump leads or jump pack. There are few ways this can be done. I would recommend using one of the first two as it would be hard to damage the car apart from blowing a fuse.
With all methods first attach the black jump lead to a good earth point, this could be door check strap or any bolts or screws you can connect to.
Note! Only get the car out of park with this method do not try to jump start through the fuse box, use the normal jump start method.
Many cars have fuse boxes in the boot which are easy to access, if you attach the black jump lead to a good earth point then with a jumper wire fitted with a fuse power up the car from the red jump lead and any fuse (not lights) turn on ignition (don't try to start) with foot on brake move gear lever into neutral, If ignition does not come on try another fuse.
Picture shows Porsche which has a power point for opening the remote controlled bonnet release. More Porsche information.
You can power up with the cigar lighter the center connection is the live, use an insulated probe or screwdriver insulated with tape to avoid shorting with outer earth ring.
You can power via the interior light but you must be very careful not to power up the earth or timer wires as a guide the earth is normally connected to the switch.