I have read on Personal Finance & Money that you can keep any and all of the 401(k)s or other options that you've had through previous employers.
But how are you able to transfer them all into one? (Especially from years and years ago.)
The simplest way to consolidate the funds your old 401(k) plans is by doing what's called a Direct Rollover (whereby the funds go directly into the new plan and skips you completely) from each of the old plans into either an IRA that you establish with a provider of your choice or even into your current employer's 401(k) plan if that is available. That way, the funds are in one central account and available to invest. Plus it eliminates the mandatory 20% withholding if the rollover is indirect and is sent to you first before the deposit into the new plan. It is important to bear in mind that you have 60 calendar days from the date of distribution to get the full amount into the new plan and a rollover is considered a tax reportable, but not necessarily a taxable event provided you deposit the funds within the time frame allotted.
Every plan administrator has their own procedures for rollovers.
In any case, you would start by browsing their website or calling them seeking information on rollover.
You will need to arrange it with both your current and prior administrators.
Usually the administrator will send the money directly to your current plan provider, keeping you out of the chain and minimizing any risks of tax complications. It may happen, though, that they have to send the check to you. In that case you will have a limited amount of time to provide it to your current plan.
You should ask your broker of choice for paperwork to move funds to them. You can't move into an account that doesn't exist, so when I wanted to move my money from an old pension plan to an IRA I set up the IRA with the broker first. When I told them it was to receive this money, they weren't asking for any initial deposit. You then have a broker and account number to give the old company to set up the move.