Inscribing is found in the weapons section of the lore tab at the bottom of the sidebar of the mysterious book, and can explain it better than me. You'll need to use the grimoire inscriber that you can make in the inventor's table; the lore page doesn't actually specify that but assumes you're using it anyways.

You can make magic pages in the anvil (or one of its upgraded forms, I am not sure) that count as either the main or alt abilities for a grimoire until post-inscription, but they're random in what ability they become. If you're looking for a specific spell, your best bet might be to find a grimoire that has that ability already, then rip out the spell or replace the other spell with another one you prefer.

From what I can tell, the level of the grimoire is based on the average level of the constituent spells. I have yet to test whether or not they get upgraded to the same level or if there's some balancing that occurs, but my assumption is that both spells are raised up to the same level, so even if you have a level 1 and a level 3 spell, upgrading them with violium (or what have you) will raise both of them to violium's level. Seeing as you can jump straight from level 1 to level 6, it seems likely.

That's about the extent of my knowledge, I just stick to old faithful Ferozium Staff.
Hope I helped.