ZTE Blade A462 Download Mode

Hi all
I'm trying to install TWRP on my ZTE Blade A462. I've been told to put the phone into download mode to do this. The way to do it is pressing volume up and down and the power key at the same time but nothing happens when I try that. Volume up and power starts it in recovery mode. Volume down and power just vibrates the phone and starts it up to a black screen where I cant do anything apart from force turn it off.
Need some help
Thanks

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