I ignored it as much as possible. If safe to do so. I'd do things like step over them to get something, or wander into another room, singing to myself, or talk normally to them like I hadn't noticed they were having a meltdown...worst thing you can do is pander to it, imo, or try to talk them round, or try to bribe them. You'll create monsters who use tantrums to control you!!
If they were physically attacking me I used to do the old wrap'n'tuck (wrap my arms and legs around them and tuck my head down into their back so they couldn't throw their head back and bang me!)
If they were in meltdown and I needed to move them, I would pick them up, facing away from me, with one arm round the top half, pinning their arms and the other holding the legs together just under the bum iyswim, and carry them and plonk them down where they needed to be and chat like I hadn't noticed/walk away/step over them/go do something else/whatever.
I normally pretended I couldn't hear the screams, and just went onto autopilot, was kind of robotic. Poker face, calm manner, talking normally.
Now they are older and understand language, I can say things like "I can see you are frustrated, but we have to do X/you must do Y, Z has to happen, etc"
Meltdowns are stressful. In order to cope, it is sometimes essential to entertain yourself. I sometimes covered their mouth with my hand repeatedly and their screams made "waa-waa-waa-waa" noises (like when you're playing cowboys and indians). Sometimes I joined in the screaming and sometimes I would burst into song..."THHHHHHHHHHHHE sun has got his hat on HIP HIP HIP HOOOOOORAY.." - they were