how do I install a utilitech model TM-097 timer on a 3 way switch circuit for my garage extierior side lights?

I have same situation. Place second switch in on position, dead end traveler (usually red) with a wire nut at first switch. Connect timer load out to other wire. 3 way is no longer functional but it works.

No you would need a 3-way timer .this is a single pole timer only . which means you are replacing a switch that's at one location only and cannot be switched on or off from any other location .