i've done this quite a few times
difficult to give definate suggestions for every situation but the last time went along the lines of....
to fit 20mm round led lights into the triangle wooden part so lamps light across the stairtread.
20mm hole cutter through wood, small half inch chase into plaster above to feed l/v cables though(you'll find a one inch gap behind wood)
do this as and where lamps required bringing all the cables up to the landing,
dropped cables in surface trunking down inside cupboard understairs to socket (for transformer) fed from cu (also understairs)via sfu rated at 3A
1.0mm twin and earth from sfu to original double switch in hall, changed to triple switch, single switch upstairs changed to double, 3core for two way switching between switches
make sure sockets labelled at 3A max and not suitable for general use.
ive seen these done with sfu understairs for switching but if possible it makes the job look far more professional if switched along with landing and hall lights
i've also had these lamps fail which is a nightmare, so dont for a second think they last forever
repairs can be made by removing lamp and carefully crimping a new lamp on snipped cable