1. Copy the "4xxxxxx" folder from the 3DS TWL nand to the DSi nand (in the 300004 folder, i could be off a couple zeroes), delete the 0000000.tmd file and cmd folder. Make sure you copy over both the "content" and "data" folders
2. Look up the title ID in that giant TMD archive, cut off the end so HxD says the length is 208, save and rename to title.tmd
3. Look at offset 1E7 on the tmd, rename the .app's last digit to the last digit listed.
4. Decrypt any ticket in the ticket/300004 folder, change title ID to your injected app's title id, encrypt ticket and save. Note that I was able to install a few games that were never preloaded on a DSi by modifying the DSi Browser ticket, so it doesn't seem that there's a lot of verification going on there.