There r 2 stations in SS 15 LRT & KTM. No MRT there.
For Kajang --> SS 15, u have 4 options:
A. KTM Kajang --> KTM KL Sentral (change platform) --> KTM Subang Jaya
B. KTM Kajang --> KTM KL Sentral (exit to change line) --> LRT KL Sentral --> LRT Subang Jaya or LRT  SS 15
C. MRT Kajang --> MRT Pasar Seni (do not need to exit station) --> LRT Pasar Seni --> LRT Subang Jaya or LRT  SS 15
D. MRT Kajang --> MRT Muzium Negara (exit to change line) --> LRT KL Sentral --> LRT Subang Jaya or LRT  SS 15

I would recommend Option C as you don't need to exit the station, so it should be cheaper than Option D.
If you opt for Option A & B, good luck to you with KTM's schedule which never fails to be cancelled or delayed.

Which part of SS 15 do you want to go to TS?

You can take KTM Komuter all the way to Subang Jaya station-interchange at KL Sentral, no need to come out.

Then from Subang Jaya KTM Komuter station there is a free bus to SS 15

Aiyo why recommend TS take much slower KTM Komuter when there's faster MRT Sungai Buloh-Kajang line and Kelana Jaya LRT line already extended to SJ area many years ago.

Edit: Since TS wanna go Hong Leong SS15 branch better take MRT Kajang station than change to KJ LRT line at Pasar Seni and then take LRT until SS15 station.

I see, because KTM cheaper so just an alternative for TS. Anyway that part of SS15 that TS wants to go is quite far from SS15 station. If me, Subang Jaya station nearer, then walk up to Aeon Big, all the way up. SS15 station to there is one big round.

With the time you spend on switching between mrt and lrt why not just take grab?