you dont, as far as i know
Ibanez Jem 777BK, Carvin DC727, Fender MIM Strat
Mesa Boogie DC5
Xotic BB Preamp, Ts9, BBE GS, Boss DD-6
Weber Mass
Morley Bad Horsie 2
Ask me any questions about my gear!
it needs to have an external speaker jack...make sure the resistance (ohms) of your amp meets the resistance (ohms) of your cabinet. make sure you dont ever turn on your combo amp without a speaker attached, though. you'll be sorry.
check the back of your combo amp for the external speaker jack if it is there your off to the races. Please check resistance ratings as mentioned above.
If you do not have an external speaker jack you can still do it but you will have to access the the back of your speaker and join a wire to the existing plate for neg + pos then wire the opposite ends into a 1/4 jack and plug it in.
The only problem with doing this is you may end up completely overpowering the existing speaker.
pics of gear updated on profile 11/16/09
I've seen it done. Most combos have an output jack. Never tried it myself.

But I have learned the hard way, not to plug a 120W Crate PowerBlock into a 30W Spider II combo. Oh how naive and... stupid I was two years ago... not even sure what I thought it would acomplish.
Heads will roll. Throats will be slit. Blood will flow like springs of water.
I hook my 30watt Zinky Blue Velvet up to a Mesa Recto 212 cab for massive bittom end. The cab is rated at 100watts. It has an external out and ya cant blow the Zinky by mismatching impedience. mafropetee, Dude did it catch fireor just blow out.
R7, SG, Texas Special
EMG modded special addition Fat Strat
Zinky Blue Velvet
other pedals and gear
Is it possible for me to do it with an 80 watt, 8 ohm Marshall valvestate combo and an 8 ohm Marshall cabinet (does the cabinet's wattage matter?)