Here's my Python script, it requires Python 3 and requests: -

from argparse import ArgumentParser, Namespace
import re
import requests
import time


def get_arg_parser() -> ArgumentParser:
    parser = ArgumentParser(
        description="Copy community follows from one Lemmy instance to another"
    )
    parser.add_argument(
        "--source-url",
        dest="source_url",
        type=str,
        required=True,
        help="Base URL of the source instance from which to copy (e.g. https://lemmy.ml)"
    )
    parser.add_argument(
        "--dest-url",
        dest="dest_url",
        type=str,
        required=True,
        help="Base URL of the destination instance to which to copy (e.g. https://lemmy.world)"
    )
    parser.add_argument(
        "--source-jwt",
        dest="source_jwt",
        type=str,
        required=True,
        help="The JWT (login token) for the source instance"
    )
    parser.add_argument(
        "--dest-jwt",
        dest="dest_jwt",
        type=str,
        required=True,
        help="The JWT (login token) for the destination instance"
    )
    return parser


def parse_args() -> Namespace:
    return get_arg_parser().parse_args()


def get_followed_communities(args: Namespace) -> list:
    print(f"Fetching list of followed communities from {args.source_url}...")
    res = requests.get(
        f"{args.source_url}/api/v3/site",
        params={
            "auth": args.source_jwt,
        }
    )
    res.raise_for_status()
    res_data = res.json()
    if not res_data.get("my_user"):
        raise Exception("No my_user in site response")
    if not res_data["my_user"].get("follows"):
        raise Exception("No follows in my_user response")
    return res.json()["my_user"]["follows"]


def find_community(name: str, args: Namespace) -> dict:
    res = requests.get(
        f"{args.dest_url}/api/v3/community",
        params={
            "name": name,
            "auth": args.dest_jwt,
        }
    )
    res.raise_for_status()
    res_data = res.json()
    if not res_data.get("community_view"):
        raise Exception("No community_view in community response")
    return res_data["community_view"]


def follow_community(cid: int, args: Namespace) -> dict:
    res = requests.post(
        f"{args.dest_url}/api/v3/community/follow",
        json={
            "community_id": cid,
            "follow": True,
            "auth": args.dest_jwt,
        }
    )
    res.raise_for_status()
    return res.json()


def get_qualified_name(actor_id: str):
    matches = re.search(r"https://(.*?)/(c|m)/(.*)", actor_id)
    if not matches:
        return actor_id
    groups = matches.groups()
    if len(groups) != 3:
        return actor_id
    return f"{groups[2]}@{groups[0]}"


def sync_follow(follow: dict, args: Namespace):
    qn = get_qualified_name(follow["community"]["actor_id"])
    while True:
        try:
            community = find_community(qn, args)
            print(f"Subscription to {qn} is {community['subscribed']}")
            if community["subscribed"] == "NotSubscribed":
                print(f"Following {qn} on {args.dest_url}...")
                follow_community(community["community"]["id"], args)
            break
        except requests.exceptions.HTTPError as ex:
            if ex.response.status_code >= 500 and ex.response.status_code < 600:
                print(f"WARNING: HTTP error {str(ex)}: trying again...")
            else:
                print(f"WARNING: HTTP error {str(ex)}")
                break


def main():
    args = parse_args()

    try:
        follows = get_followed_communities(args)
    except Exception as ex:
        print(f"ERROR: unable to fetch followed communities from {args.source_url}: {str(ex)}")
        return

    print(f"Syncing {len(follows)} followed communities to {args.dest_url}...")
    with open("failures.txt", "wt") as failures:
        for follow in follows:
            try:
                sync_follow(follow, args)
            except Exception as ex:
                print(f"ERROR: {str(ex)}")
                failures.write(
                    get_qualified_name(
                        follow["community"]["actor_id"]
                    )
                )
            time.sleep(1)


if __name__ == "__main__":
    main()

You use it like this (for example), assuming it's saved to sync.py: -

python sync.py --source-url=https://lemmy.ml --dest-url=https://lemmy.world --source-jwt=abc123 --dest-jwt=bcd234