Hi buddy,
My current WIP is a Tamiya kit and comes with both link & length (L&L) tracks and one piece tracks. If it's a German or Russian build you'll want to use the L&L tracks so you can obtain some realistic sag. I only use the one piece tracks on armour with 'live' tracks like the Sherman where the tracks are very tight and don't sag.
Personally, I never glue the tracks onto the model. I assemble the top and bottom runs and use the single links to attach them being careful not to glue the tracks to the wheels. I leave it to so the very last joint is left unglued so after they have been left 'in-situ' & have fully set they can be fully removed in one piece for proper painting along with the wheels.
If you need to add sag to the top run I'd recommend packing outthe top track run between the return rollers with foam or foam ear plugs etc whilst the glue is still setting-this will push down on the tracks where the sag needs to be, just make sure you use enough links to compensate for te sag and don't leave yourself short of link for the other side!
This is my current WIP - see what I mean? It makes everything so much easier:
For any delicate work like this, particularly on track links, I recommend the use of Tamiya's Extra Thin Cement that you see in the square bottle at the back of this photo. It's very easy to use and leaves little mess.
Best of luck and remember - take your time and don't rush. Make sure you triple check your links against the instructions before reaching for the glue as it's VERY easy to get them wrong. Glue link and lengths in the wrong direction and you'll probably struggle. Been there and done that!
I hope this helps!
Kind regards,
Ben