which part are you struggling with? mostly I found it’s just putting in your current investments, your savings rate, and expected spending rate after retirement, then tweak the numbers until things work

I have no real way of estimating my expected spending rate after retirement. Maybe 3 of my kids will want to be supported in Kollel (and I'd love to do that for them for a bit) and maybe they will have 13 kids and live down the block and come for large Shabbos seudos, and maybe we will make aliyah when we retire, and maybe my special needs son will need full time care, and maybe...