You have to have a Formula to make a scroll, and have to be able to provide a casting of the spell, and have Magical Crafting feat. There is somewhat widespread debate on how specific the formula has to be.
Some readings imply you have to have a (bought or found) formula for as specific as '1st level heal spell'. Others suggest you have to simply have a formula for '1st level spell' and can then create a scroll for any first level spell. Others suggest it might be a formula for '1st level common spells' and uncommon spells might require a separate formula.
Assuming you have the spell availability, and the formula requirement (however specific), and the magical crafting feat. You spend half the cost of a scroll and craft for 4 days(although you can do a batch of scrolls, I think 4 but I could be wrong, if you do a batch, you multiply the cost by the number in your batch up to the max you can do in a batch). Make you crafting check, at that point after your 4 days, you can either pay the rest of the cost of the scroll in gold. (the rest of the other half you haven't paid yet) Or you can choose to take more time, and earn income off the crafting table to reduce the cost of the remainder of the scroll by the amount per day indicated by the chart, going off your level, your skill rank, and how well you did (critical boosts your effective level).
Then you have your scroll to carry into the dungeon as an emergency spell.