Install a "user script" plugin for your browser, and click on "raw" bellow... Then go to twitter and follow the followers of someone. And then after a while unfollow all those you have never retwitted.

twitter-tools.user.js

// ==UserScript==
// @name        twitter-tools
// @namespace   lexoyo.user.scripts
// @description Add buttons twitter to gain useful real followers
// @include     https://twitter.com/*
// @version     1.1.4
// @run-at      document-end
// @grant       none
// ==/UserScript==

setTimeout(start, 1500);
var ps = ps || window.history.pushState;
window.history.pushState = function() {
    ps.apply(this, arguments);
    setTimeout(start, 1500);
};

function start() {
    addCss (
        '.toubon { \
height: 18px; \
float: right; \
padding: 9px 10px 9px; \
color: #66757F; \
min-width: 65px; \
background-color: #F5F8FA; \
background-image: linear-gradient(#FFF, #F5F8FA); \
background-repeat: no-repeat; \
border: 1px solid #E1E8ED; \
border-radius: 4px; \
cursor: pointer; \
display: inline-block; \
font-size: 14px; \
font-weight: bold; \
line-height: normal; \
position: relative; \
margin: 12px 140px 0 0; \
}'
    ); 
    console.log('start');

    if(window.location.href.indexOf('/followers') > -1 || window.location.href.indexOf('/search?f=users') > -1) {
        addButton('followall', 'Follow all', function (event) {
            event.stopPropagation();
            followAll();
            srollToBottom(followAll, 10000);
        });
    }
    else if(window.location.href.indexOf('twitter.com/following') > -1) {
        addButton('unfollowLoosers', 'Unfollow loosers', function (event) {
            if(localStorage.getItem('loosers')) {
                event.stopPropagation();
                unfollowLoosers();
                srollToBottom(unfollowLoosers, 10000);
            } 
            else {
                alert('Click on "find loosers" button first');
                return;
            }
        });
    }
    else if(window.location.href.indexOf('/following') > -1) {
        addButton('followall', 'Follow all', function (event) {
            event.stopPropagation();
            followAll();
            srollToBottom(followAll, 10000);
        });
    }
    else /* if(window.location.href.indexOf('/with_replies') > -1) */ {
        addButton('findLoosers', 'Find loosers', function (event) {
            event.stopPropagation();
            findLoosers();
            srollToBottom(findLoosers, 1000);
        });
    }


    function addButton(id, label, onClick) {
        if(document.body.querySelector('#' + id)) return;
        var stopScroll = false;
        var selector = '.ProfileNav-list';
        var buttonHtml = '<div class="toubon" id="' + id + '">' + label + '</div>';
        var el = document.body.querySelector(selector); 
        if(!el) el = document.body.querySelector('.AdaptiveFiltersBar-nav'); 
        el.innerHTML += buttonHtml;
        var followall = document.getElementById(id);
        followall.addEventListener('click', onClick);
    }


    function unfollowLoosers() {
        var retweeted = {};
        if(localStorage.getItem('loosers')) {
            retweeted = JSON.parse(localStorage.getItem('loosers'));
        }
        var unfollowed = [];
        var kept = [];
        $('.ProfileCard').each(function(){
            var login = $('.u-linkComplex-target', this).text()
            if(retweeted[login] !== true) {
                unfollowed.push(login);
                $('.user-actions-follow-button', this).click()
            } else {
                kept.push(login);
            }
        });
    }
    function findLoosers() {
        var retweeted = {};
        if(localStorage.getItem('loosers')) {
            retweeted = JSON.parse(localStorage.getItem('loosers'));
        }
        $('.username b').each(function() {
            if($(this).text() != '') retweeted[$(this).text()] = true;
        });
        localStorage.setItem('loosers', JSON.stringify(retweeted));
    }
    function followAll() {
        $('.ProfileCard .user-actions.btn-group.not-following').each(function () {
            if (!$(this).hasClass('can-dm') && 
                !$(this).hasClass('protected') && 
                this.innerHTML.indexOf('default_profile_images') === -1) {
                $('.user-actions-follow-button', this).click()
            }
        });
    }
    
    var stopScroll = false;
    function srollToBottom(cbk, delay) {
        var top = $(window).scrollTop();
        console.log('srollToBottom start at ', top);
        $(window).scrollTop(3000000);
        setTimeout(function () {
            console.log('timeout', top, $(window).scrollTop() !== top);
            cbk();
            if (!stopScroll && $(window).scrollTop() !== top) {
                srollToBottom(cbk, delay);
            }
            else alert('The End Of Scroll.', top);
        }, delay);
    }




    function addCss(cssString) {
        var head = document.getElementsByTagName('head')[0];
        var newCss = document.createElement('style');
        newCss.type = "text/css";
        newCss.innerHTML = cssString;
        head.appendChild(newCss);
    }

    console.log('end');
}