Adding a stay alive capacitor to a QSI Titan U type decoder is easy. Simply solder the common blue wire of the stay alive capacitor to the common blue wire of the decoder and the remaining black wire to the Titan's small-cap black wire. Some of the most popular stay alive caps we recommend are from TCS and SoundTraxx:

TCS 1668 KA4 KeepAlive
TCS 2000 KA3 KeepAlive
SoundTraxx 810140 CurrentKeeper

QSI Titan U Type Decoder with Stay Alive Capacitor