Well, that page has gone 404 since yesterday, so here's the text, rescued from Google's cache...
Merging new spells in with the d20 Spells module
To make a library spell module that merges in the d20 spells with any new spells you wish to add:
Copy d20spells.mod to the desktop
Change the file extension from .mod to .zip
Unzip d20spells.zip
There are three files in d20spells: client.xml, definition.xml and thumbnail.png. Open definition.xml with notepad (or your editor of choice). It has six lines. Change the <name> from "d20 Spells" to "d20 Spells Mine" (or whatever), then save the file and exit.
Open the client.xml file. If you do not have a better editor, use notepad. It will probably take a while to open. On the fifth line, that starts <name type="string">, change the name to exactly what you changed the name to in the definition file (my suggestion was "d20 Spells Mine"). It must match exactly, including case.
We need to keep the spell lists from doubling up when they're displayed in the Library. So change the fourth line of the client file from <d20spells static="true"> to <d20spellsMine static="true"> (or whatever). You also need to change the change the closing element (in line 65, right above </library>) to match, from </d20spells> to </d20spellsMine>.
Next we need to give the lists of spells a unique name. To do that, we are going to find and replace every instance of "spelllists" with "spelllistsMine" (or whatever name you want). When mass replacing it’s always a good idea to make a copy of the file beforehand, because you can really mess a file up with a mistake or typo and it’s often easier to start over than repair the mess.
In the last step, we broke the links in the <library> entries, since they are now referring to "spelllistsMine", but still pointing at "d20 Spells". To fix that, for each <library> entry, from <bard> to <domains>, change the end of the <recordname> from "@d20 Spells" to "@d20 Spells Mine". The library links now point to the lists in this module. Make sure you don't change any other instances of "@d20 Spells" - only those between the <library> and </library> tags.
Now find the description of the first spell, <acid arrow>. Move your cursor to the start of the next spell under it, <acid fog>. Scroll down to the end of Zone of Truth, but before the closing </spelldesc> tag. Hold down shift and click at the end of </zone of truth>. You should now have every spell description except the first selected. Press delete to get rid of them, then save the file.
What have we done here? We've made a shell of the old book. If you build the new library module now, start FG and activate both d20 Spells and d20 Spells Mine, when you mess with d20 Spells Mine you'll find you can still open spells and get spell descriptions. What? Didn't we just delete all the spell descriptions? We did, but we didn't delete the links in the spell lists that follow.
If you deactivated the d20 Spells module, you wouldn't get the spell descriptions any more (and in the chat window you'd get a message saying something about the d20 Spells module being inaccessible). What has happened is the spell links in the spell lists still point at d20 Spells, and as long as that library book module is activated all the old spells will be accessible.
The next step is to add your spell(s) to your new book. We kept Acid Arrow as an example, so start by changing all the elements of this to your first new spell. If you're feeling confident, copy and paste the spell template and keep adding new ones, otherwise carry on with the other steps. You can always come back and add more spells later.
Next we need to add the new spell(s) to the class lists. Let’s say I was adding Greater Mage Armor, a 3rd-level Sor/Wiz spell, and I’ve already input it into the <spelldesc> element, where I tagged it as <magearmorgreater>. Now I go down to <spelllistsMine> and find the nested <wizard> group which holds the <level3> spells. I want to insert my new spell in here. I’m going to follow the PH convention and insert the spell in the M's listing, not the G's. I scroll down and find where Greater Mage Armor should go - between <lightningbolt> and <magiccircleaginstchaos>. I copy and paste the <lightningbolt> entry so I have two copies of the entry, one after the other. Then I use the second <lightningbolt> entry as a template to make the link to the new spell, Greater Mage Armor. To do this, I change the element names from <lightningbolt> and </lightningbolt> to <magearmorgreater> and </magearmorgreater>. Next, I change the <recordname> to point at the new spell. In this example, I change "spelldesc.lightningbolt@d20 Spells" to "spelldesc.magearmorgreater@d20 Spells Mine". Basically, we have just created a link to the spell description in this module, like we did with the library links in step 8, above.
Copy and paste this new link into any other class lists, wizard school lists or cleric domains you need to add the spell to. (In the example above, that would include the <level3> section of the <conjurationschool> of the <wizardschools> lists.) Make sure you adjust the level number if the spell has different levels in other classes.
You can now save client.xml and zip the three files back together. It is important you zip only the three files and not include the folder the files are in. Rename the zip file "d20spellsmine" (or whatever) and change the .zip extension to .mod, then move it to the "modules" folder in your FG Application Data folder.
You're now ready to run FG and play with your new spellbook!