First, buy this: https://www.amazon.com/dp/B004FD1D2C/ref=cm_sw_r_cp_api_NwYOxbJ1RWS6S
And also buy a CompactFlash memory card or an SD memory card plus a CF adapter.
Then, use this guide (or Google for your specific model) to complete the surgery: https://www.ifixit.com/Guide/iPod+5th+Generation+(Video)+CF+or+SDHC-SDXC+Memory+instead+of+HDD+Replacement/7492
After that, just restore the iPod with iTunes. If the parts were installed correctly, iTunes will automatically recognize the new storage capacity. Then, just sync and you’re good to go!