How to feed up a runt?

[200xth]

How should I go about getting her up to size? I'm considering offering her another ~18g mouse in 5 days time. But how should I proceed from there?

You don't need to do anything special to get her up to size, just feed her appropriately sized meals on a regular basis; she'll start putting on weight consistently. A lot of people do 5 or 7 day schedules, and either will work. Pick whatever is convenient for you. If you can rotate feeding days easily and want to do a 5 day schedule do that. If feeding on a specific day works better for you, do 7 days.

As long as you're getting decent meals into her on a regular basis she'll put on weight for you.