Formula feeding - confused first time mum!

Liz

Unfortunately my baby can’t breast feed due to prematurity and a tongue tie.

I have been pumping and supplementing with formula as I just can’t keep up with the demands of pumping and tube feeding my baby whilst we are waiting for him to learn to take a bottle. The hospital advised to make formula feeds up when needed and not store these as the powder is not sterile. However, I’m confused by how I would make a feed up in advance if I take my baby out?