A magic-user finds a scroll with a spell on it, or else he finds another magic-users spell book. How does he transfer a new spell from this source into his own personal spell book? To begin with, the magic-user selects a new spell, then must cast a read magic spell, as that spell is needed to read magical writing written by Now the magic-user must roll an intelligence check (1st Edition Players Handbook, page 10, Intelligence Table II, Chance to Know Each Listed Spell) to see if he is able to know and understand the spell. (Remember that a PC is only able to know a limited number of spells of each level if his intelligence is below 19.) If the magic-user fails his chance to know the spell or has already reached the maximum number of spells he can know at that level, proceed to the section on Recording unknown spells.
Assume that the magic-user made his intelligence check and has not yet reached the maximum number of spells he may know at the level in question. He has also used his read magic spell, and he has read and understood the spell he picked; he is now ready to record the spell into his own book. To write the spell, the magic-user needs an unused quill and an unopened bottle of ink. The ink costs 100 gp per spell level of the spell being inscribed (Unearthed Arcana, page 79). A spell takes one turn per spell level to inscribe into the magic-users spell book. The magic-user gathers his materials, sits down, and eventually has a new spell in his book. (In the 2nd Edition game, it takes 1-2 days per spell level to inscribe a new spell into a spell book.)
Dragon Magazine 147, p34-35