In a new macOS terminal run `bash ./futurerestore.sh <path/to/SHSH> <path/to/ipsw> -c` and follow instructions

futurerestore.sh

#!/usr/bin/env bash
#
# Perform iOS and iPadOS downgrade using gaster and futurerestore.

set -e

BOLD=$(tput bold)
readonly BOLD
NC=$(tput sgr0)
readonly NC

# register the cleanup function to be called on the EXIT signal
trap cleanup EXIT

function err() {
  echo "[$(date +'%Y-%m-%dT%H:%M:%S%z')]: $*" >&2
}

function is_command_installed() {
  if ! command -v "${1}" &> /dev/null
  then
      err "${1} could not be found"
      exit 1
  fi
}

#######################################
# Deletes the temp directory.
# Globals:
#   WORK_DIR
# Arguments:
#   None
# Outputs:
#   Writes location to stdout
#######################################
function cleanup {
  # shellcheck disable=SC2317
  rm -rf "$WORK_DIR"
  # shellcheck disable=SC2317
  echo "Deleted temp working directory $WORK_DIR"
}

function create_tmp_dir() {
  WORK_DIR=$(mktemp -d)
  readonly WORK_DIR

  # check if tmp dir was created
  if [[ ! "$WORK_DIR" || ! -d "$WORK_DIR" ]]; then
    err "Could not create temp dir"
    exit 1
  fi
}

function show_manual_action() {
  echo
  echo "${BOLD}${1}${NC}"
  # shellcheck disable=SC2162
  read -p "Press enter to continue"
}

#######################################
# Detect the architecture of current
# macOS.
# Arguments:
#   None
# Outputs:
#   Writes an error msg if the current
#   arch is not supported
# Credit:
#   palera1n team
#######################################
function detect_arch() {
  local arch_check
  arch_check=$(uname -m)

  case "${arch_check}" in
      x86_64*)
          arch=x86_64
      ;;
      arm64*)
          arch=arm64
      ;;
      *)
          error "Unknown or unsupported architecture ($arch_check)."
          exit 1
      ;;
  esac
}

function main() {
  is_command_installed 'git'
  is_command_installed 'cmake'
  is_command_installed 'gmake'
  is_command_installed 'xcrun'

  create_tmp_dir
  pushd "${WORK_DIR}" || exit 2

  local download_version
  download_version=$(curl -s "https://cdn.nickchan.lol/palera1n/artifacts/c-rewrite/main/" | awk -F'href="' '!/\.+\// && $2{print $2}' | awk -F'/' 'NF>1{print $1}' | tail -1)
  detect_arch
  curl -o 'palera1n' -L "https://cdn.nickchan.lol/palera1n/artifacts/c-rewrite/main/${download_version}/binaries/palera1n-macos-${arch}"
  chmod +x ./palera1n
  set +e
  xattr -d com.apple.quarantine ./palera1n
  set -e

  git clone --recursive --depth=1 -j8 https://github.com/0x7ff/gaster.git gaster
  pushd ./gaster || exit 3
  gmake
  popd || exit 4

  if  [ "${3}" = "-c" ]; then
    git clone --recursive -j8 -b 'cryptex' 'https://github.com/futurerestore/futurerestore.git' 'futurerestore-src'
    curl -o 'deps.tar.zst' -LO "https://cdn.cryptiiiic.com/deps/static/macOS/${arch}/macOS_${arch}_Debug_Latest.tar.zst"
    tar xvvf ./'deps.tar.zst' -C ./futurerestore-src/dep_root
    pushd ./futurerestore-src || exit 5
    git cherry-pick 9554c0068dc50e141872ced5da2bd95baa595805^..36879969be71d56af062aa99be5f28ee482a12bc
    cmake -DCMAKE_BUILD_TYPE=Debug -DCMAKE_MAKE_PROGRAM="$(which gmake)" -DCMAKE_C_COMPILER="$(which clang)" -DCMAKE_CXX_COMPILER="$(which clang++)" -DCMAKE_MESSAGE_LOG_LEVEL="WARNING" -G "CodeBlocks - Unix Makefiles" -S ./ -B cmake-build-debug -DARCH=${arch} -DNO_PKGCFG=1
    gmake -j$(($(/usr/sbin/sysctl -n hw.logicalcpu) + 1)) -l4 -C cmake-build-debug
    popd || exit 6
    cp -v ./futurerestore-src/cmake-build-debug/src/futurerestore ./
  else
    curl -LO https://nightly.link/futurerestore/futurerestore/workflows/ci/main/futurerestore-macOS-DEBUG.zip
    unzip -p futurerestore-macOS-DEBUG.zip | tar -xv
    rm -v futurerestore*.zip
    set +e
    xattr -d com.apple.quarantine ./futurerestore
    set -e
  fi

  # RELEVANT PART
  ./palera1n -l -D
  sleep 2
  ./gaster/gaster pwn
  ./gaster/gaster reset
  sleep 2
  # --skip-blob is useful if you restore using an OTA update SHSH blob
  if  [ "${3}" = "-c" ]; then
    # WORKAROUND from https://gist.github.com/mineek/bd8d0e002ce67e82831a23a8d7eceb3c by Mineek
    ./futurerestore -t "${1}" -d --no-cache --use-pwndfu --latest-sep --latest-baseband --skip-blob --set-nonce "${2}"
    show_manual_action "Force reboot your iDevice using the button combination."
    ./palera1n -l -D >>/dev/null 2>&1 &
    if command -v "irecovery" &> /dev/null; then
      echo
      echo "${BOLD}Waiting Recovery Mode...${NC}"
      set +e
      while :; do
        if [[ "$(irecovery -m 2>/dev/null)" =~ "Recovery" ]]; then
          break
        else
          sleep 1
        fi
      done
      set -e
    else
      # irecovery not found! Switch to manual mode (legacy).
      show_manual_action "Wait Recovery Mode logo."
    fi
    kill %1
    ./futurerestore -d -t "${1}" --latest-sep --latest-baseband "${2}"
  else
    ./futurerestore -t "${1}" -d --no-cache --use-pwndfu --latest-sep --latest-baseband --skip-blob "${2}"
  fi

  popd
}

main "$@"
exit 0