Sometimes they won't take to a bottle if they're still suckling mum, but if they will, I find topping up in the field the best option, if it's practical for you.
I usually have them all indoors for a couple of days, until the lamb/s I'm topping up run to me when they see me, then move them to small paddock to make sure they'll continue to do do when they're outside. I make a call as I arrive with the bottle, which they'll learn to recognise - for me, it's "Top Uuuuuuppppssss!"