It's only giving them the correct amount of light that stops them becoming leggy. February to this time of year that's easier said than done unless you invest in grow lamps.
As to already leggy ones, depends what you have sown.
I haven't sown anything yet and won't do so for a good few weeks.

It might help to give them less warmth too.

I have managed to save leggy ones in the past simply by sifting more compost over them, worth giving it a try if there is enough room in the tray..

I started off my Cerinthes seeds far too early AGAIN..tut tut...and the went very leggy. I transplanted them into pots with some fresh compost and buried them up to their seed leaves and they are now growing well and developing true leaves, so it can be done sometimes at least...worth a try.