I'm sure there's a YouTube video out there, somewhere ... showing how it's done.

But, how I would do it would be to make a cut (back to belly) as close to the gill plate as possible, with the knife angled towards the gill plate. Then carefully run the knife (sideways) into the bottom of the belly cavity ... turn the blade down and slice open the belly. Remove ALL the innards, together, then separate the egg sac from them ... being careful to not nick the sac with the knife, or tear it when separating it from the other organs. THEN proceed to finish cleaning the fish. (but that's from the viewpoint of someone that fillets with an elec knife !! )

It's probably easier to remove the egg sac during the pre-spawn period, when the egg sac is bright yellow and fairly firm. Once it gets time for them to drop, the egg mass is mushy/runny and the sac isn't as firm and more easily torn.