Where is the Decaying Crypt?

I always hear about the Decaying Crypt, but which act is it in? How can I get access to it from a way-point?

The decaying crypt can only spawn in two possible locations in the Fields of Misery zone. The best way of finding it is by taking a look at the image provided in this guide:

As for an actual Reverse Crypt run (4 player DC run is the fastest XP in the game, currently), here is the best way of doing it:

1. Every play make their own game to look for the crypt.
2. If a player finds the crypt, the other 3 players join that game.
3. The designated crypt runner runs through the crypt, triggering all the Wretched Mothers to spit out extra zombies. They should take care to not kill anything.
4. The other 3 players, in the mean time, goes to Festering woods to prepare elites.
5. Once the runner reaches decaying crypt level 2, he teleports out.
6. All 4 players proceeds to gain 3 stacks in FW (if FW does not provide at least 3 stacks, which is rare but possible, then gain the 3rd stack in cemetary or leoric's courtyard).
7. The runner then uses his original TP back to the crypt level 2.
8. The crypt level 2 provides both a resplendent chest and an event, which takes you up to the full 5 NV stacks.
9. Finally, all 4 players goes backwards through the Crypt and clears out the entire zone.

This full run should take ~5 minutes with a good party on MP10, and can provide up to 300m xp per hour, which is the best in the game.

Also, this map on diablowiki on festering woods should help you reach 3 NV stacks as fast as possible:

Finally, for the solo xper, this is my favorite XP path in act 1:

1. Go to Festering Woods to gain NV stacks.
2. TP to cemetary of the forsaken and take the northwest exit to Fields of Misery.
3. Clear FoM using an optimal path until you either find the crypt or complete the area.
4. Clear crypt if it exists.
5. (optional) clear halls of agony level 2 (this area has pretty good xp/hour, higher than weeping hallow)
6. Start new run.

Decaying crypt is a randomly spawned dungeon in fields of misery. Check this out for some farming routes for A1 and A2.

Short version for act 1:

1. Festering woods
2. Leoric's Manor (have 5 stacks for FoM)
3. Fields of Misery
4. Decaying Crypt (if it spawns)
5. Halls of Agony level 2 (optional)

There is no 'Best farming route'. It depends on what your farming goals are.