Air handler

This should be doable: First get pen and paper-start at the furnace, theres a group of 5-8 wires leaving the furnace anfd going to the thermostat and a group of 2-3 going to the condenser outside that all need to be identified and the location in the furnace recorded if your furnace has a control board simply right the letter and the corresponding wire color:and whether it belongs to the condenser grp or the T-stat grp.

Furnace
R-red-to t-stat
C-yellow to cond
Y-red to cond and yellow to t-stat
G- green-to t-stat
W- white- to t-stat

If you dont have a circuit board youll be doing the same thing but using the components rather than letters;

Transformer 24v- red to t-stat
transformer 24v com -yellow to cond
Red from cond connected to yellow from t-stat
fan relay - green to t-stat
gas valve - white to t-stat

K then go to you t-stat and simply connect the corasponding component or leter to the thermostat gterminal

Cond Furnace Thermostat

yellow com- blue -C or B
gas valve or w-white -W
fan relay or g -green -G
trans 24v or R -red - R
red connection to cond or Y on board -Y
Hope this helps.