1) Get this (SEUS) :
Error 404 - Drivers and firmwares catalog, news, articles: X-DRIVERS.COM
2) Flash your phone with the latest software available using SEUS.
3) Go to C:\Program Files\Sony Ericcson\Update Service\db\13740270\blob_fs\
4) Copy the 2 largest files you'll find there to C:\Flashtool\leajian (I assume you created there a folder called leajian)
5) Run FlashTool
6) Click Advanced > Decrypt Files
7) Source folder : C:\Flashtool\leajian
8) From Files, select all (Click & Ctrl + A) and click -> to move the 2 files to Files to convert
9) Click OK
10) Wait patiently till it finishes...
11) From Folder list, select all (Click & Ctrl + A) and click -> to move the 2 files to Firmware content
12) On Device write the model [ex.
Xperia X10 Mini Pro(U20i)], on Version write the firmware version and/or the Android version [ex. 2.1.1.A.0.6 (Eclair_2.1)], on Branding write the region and your name if you wish [ex. Greek (by Leajian)].
13) Click OK
14) On Device Selection choose the device it is for (ex. U20i)
15) Wait very patiently...
16) When it finishes the .ftf file is located on C:\Flashtool\firmwares