I've done this on one of my old bikes, an old 16in alu GT, it's quite simple…
48/36 chain rings up front. chainrings bolt straight on to XT cranks -Works fine with deore front mech & shifters, wind the screw in on the mech so it doesn't go over to granny position.
23 or 25-13 rear cassette and shimano 105 short cage rear mech. Works fine with mtb shifters.
fit some flat bars if you want and some skinny tyres. I ran Spesh Nimbus on mine, got some Panaracer Rimbos which work really well too.
Job done.
I've done the same on a Giant Xtc but haven't bothered with the larger chain rings this time so I can use it off road too by just swapping the tyres. Riding off road with 48/36 up front is doable but it's not as nice as the usual 44/32/22. The 23-13 block was also a bit too close, 25-13 works well enough on and off road.
Kev