There are plenty of decoder installation videos on YouTube as well as magazine articles to give you ideas about ”how to .”  You might also find one that is specific to disassembly of your particular locomotive.  The only difference between Decoder Buddy and an 8 pad decoder is the size and the 21-pin Decoder fits into the 21 pin plug.

The large motherboard is one sided but to be conservative a piece of insulating (Kapton preferred) tape placed on the bottom of the Decoder Buddy before mounting if directly onto a metal surface is highly recommended.  When mounting with double sided foam tape the extra insulating tape is not required.  Electrical isolation of the electronics and the locomotive chassis is the goal.

I prefer to cut the connectors off any of the wires from the track, motor, speakers, lights or stay alive noting any polarities that are relevant.  I cut the wires close to the connectors to leave as much wire as I can for routing purposes.  Before I solder the wires onto the Decoder Buddy pads I cut them so that there is about ¼” extra.  An exception is the motor wire.  I prefer to leave it long and tuck it out of the way if possible.  The extra ¼” of wire keeps the installation neat and leaves a little play for contraction, routing or whoopses.  I also keep the leftover wire for later use.  It does come in handy!

Motor wires, motor wire colors and decoder boards have given me fits and wrong way starts more than once. Now I solder the track wires and speaker wires to the Decoder Buddy but only clamp the motor wires to the decoder buddy.  I test the direction on a program track to make sure the locomotive moves in the direction that I want it to.  If I need to change the direction of the motor I change the leads on the pads and then solder them onto the board.

There are four common ways to install the mother board.  If you are lucky you have tapped holes for screws.  The screws are most likely metric size (M3?).  Second, if you have a circuit board in place that has four tabs with soldering pads on both ends remove it.  If there is enough headroom (height) Scotch 2 sided tape works well to attach the Decoder Buddy.  The plastic clip should act as insulation if you want to use Kapton tape to secure the Decoder Buddy Directly to the plastic.  A third way is to use GOO, rubber  cement or silicone gasket sealant as a semi permanent adhesive.  The fourth way would be a DC powered locomotive like an Athearn blue box or a Proto 1000 without a printed circuit board.  After you isolate the motor electrically you can place a piece of Kapton tape on the bottom of the Decoder Buddy for “isolation insurance” then use the Kapton tape to attach it to the motor.

Read articles, blogs and posts.  Model Railroad Hobbyist Magazine and https://model-railroad-hobbyist.com/recent-posts and it’s associated search box in the upper right hand corner have a wealth of information on upgrading locomotives.  You should find the price is right too!!!