Geoff says:

In theory it’s quite simple – you cut the wires between the wheel pickups and the motor and connect the ends to the appropriate wires of the decoder.

In practice, it’s a bit harder.

Check the locomotives you have to see if they have power pickups on the wheels on both sides. Some older ones with metal chassis will have a power pickup on one side only,the power to the other side of the motor going through it’s attachment to the chassis. These can be quite difficult to convert as you need to isolate the engine from the chassis so you can make the connection through the decoder.

. Other locomotives won’t Have enough space under the body to accommodate the decoder, so you need to get inventive such as putting the decoder in the tender.

Wiring a sound decoder can be just as easy ( or hard ) as wiring in a regular decoder. Where sound gets complicated is the space issue. Sound decoders are slightly bigger than non sound decoders, but the biggest space issue is where to put the speaker. With a steam loco, the tender maybe the place to put it.

If your soldering skills are good, there’s no reason you couldn’t learn to do it yourself. There’s also a good chance someone else has already done it and written about it on the internet. One wonderful thing about model railroaders is that they love helping other people. (That can be a problem though as many of us have contradictory opinions on the best way to do something😀).

If you can get someone to guide you through the first one that will help enormously. Obviously, the first one you convert should be the one you like the least! Good luck!