I AM NOT RESPONSIBLE FOR ANYTHING IF IT GOES WRONG (it should not go wrong though)
First you will need an image. Search for a stock 2013 nexus 7 image. Than you will need to download google USB drivers :
Than install them by opening the device manager program which is installed by default on any windows computer. Plug your tablet in, right click it in device manager and press update driver software and browse for the driver you downloaded. Than install it.
Now that that is done download the android SDK.
Extract just the SDK folder (the other parts are not needed) and enter it once extracted. Click on the platform tools folder, hold shift and right click. Click on
"open new command window here"
Now put your tablet into fastboot mode. Type
"fastboot devices" in the command window. If there is something that shows up this means that your tablet is recognized.
Now type "fastboot OEM unlock" in the command window. Select yes on your tablet with volume keys and click power on your tablet. Than it will run through the unlock bootloader practice. This will erase all content on the tablet. It may take a bit.
Once that is done, put your image in the tablets internal storage.
Now download the latest twrp for your device.
Press download latest .img
Now, put your twrp image in the platform tools folder and name it something easy to type. Now, in the command window type:
Fastboot boot (name of your twrp here).IMG
This will boot into twrp. It will not install it.
Now, in twrp press install and browse for your image (must be a .zip file) and flash it. If all goes well than reboot your system. If there is a problem try another image.
You do not need to unlock bootloader or install drivers again. Just boot back in to twrp and flash another image (if it goes wrong)