Scribes may only encode a limited number of scrolls per day. Use the Number of Spells by Level, from Table 9: Magic-User Advancement (pg 13). It requires 30 minutes per spell level to encode the spells.
At lower levels the maximum number of spells per day obviously doesn't jive with how long it actually takes to encode the scroll. I apologize if this seems jarring. The plain truth is that if it is represented as taking the entire day to encode the spell slots available, then the class is screwed. It would take an entire day for a 1st level Scribe to encode a Sleep scroll, one turn to use it, then another day to encode another one. Yuck. The other side of it was to just make it a flat number of minutes per spell level and leave it at that. Scribes would be overpowered like that. The same 1st level Scribe could spend the same day encoding Sleep, except he would have a brace of Sleep scrolls at the end of the day. This is my compromise, which I narrate by saying that as the Scribe advances, his mental discipline and ability to concentrate and encode for longer periods of time.By the way, the player can pretty much call the spells in his spellbook whatever he wants. As long as it is known that Silver Tongued Devil is actually Charm Person everybody will be happy.
This is pretty much the easy part. Unless it is an invented spell, the spells are the same as in S&W Core. Some may not be particularly suited to scroll work, that is left to individual referees to determine. In any event, spells that are pronounced from scrolls have the same range, duration, etc. They are cast at the level of the Scribe who encoded them.
- Scribes may not cast spells directly from their spellbooks. The information in the book describes how to encapsulate magical power in a scroll. It is not actually a spell. Likewise, spells encoded to scrolls may not be transcribed into the Scribe's spellbook.
- Scribes begin with 3 or 4 spells in their spellbooks. Any other spells must be acquired through play. They do not automatically add spells as they gain levels.
- All other class information, such as HD, XP, and so forth, is identical to the Magic-User.
That's it for now. I'm sure you noticed that I didn't address clerical magic. I'm not sure how it fits into this paradigm. I'm thinking on it, though. I'm also thinking of something similar, except with potions. I've run into some potential issues with that one already, and it's still in the conceptual stages. I think it could be neat if I can work it out, though.