There are 2 options. Go via Santa Terresa (if roads open) or go via Ollantaytambo.
Option 1 via Santa Terresa
-Get a collectivo to Santa Terresa for 50 solas or $16 US
-Next step has two options.. walk two hours on a train track that supposed to be beautiful but can be dangerous if it gets dark (another option is staying in a town along the walk). There is no permit needed to talk this walk. Or take the train for a 20min ride.
If your wanting a train ticket buy it here in Santa Terresa. You want Hidroelectrica train station to Aguas Calientes. $8 - $18 US. Be warned Hidreoelectrica isnt a train station its more of a stop in the middle of no where next to a Hydroelectric dam.
-Either way you will need to get a taxi ride from Santa Terresa to the Hidroelectrica Train station - a few dollars and about 20-30min inc a small walk.
-Arrive in Aguas Calientes and walk/hike 1 hour 30min up to Machu Picchu $46 US entry fee.
- Alternative (take the bus up or back down) $9pp US each way (Rip off)
Whole thing there and back was going to cost $94 US pp
However they have had bad weather and the road from Santa Maria to Santa Terresa (which you have to take) was closed due to mud slides and is very dangerous. This happens a lot Dec-Mar (wet season)..
2 day tour
Leave Cusco at 2pm catching a bus/collectivo ($3US pp) to Ollantaytambo (you could stay here a night its beautiful). Scenic train trip with glass roof from Ollantaytambo to Aguas Calientes. Make sure it is Inka Rail as its the only company to refund.. MP is constantly closed due to bad weather so this is a must.( Peru Rail didn't refund last week during bad weather but they are cheaper $35pp compared to $40 but Inka Rail looked much nicer) Then stay the night at a hostel ($15 US pp). Wake up and enjoy the day at Machu Picchu and maybe see the sunrise there. Return the same way at 2pm and be back in Cuzco for 6pm.
Can do it by yourself if you can find cheap train tickets for around $170 US or find a good tour operator.
You can find some tours for $170 US... (most others were $195 - $230)
WARNING: Be careful with your tour operators. These guys suck the nostalgic blood from the veins of Machu Picchu and will leave you high and dry. Especially avoid Mapi Trek Peru (near Jacks Cafe). They left us and 8 other people at ollantaytambo with no way to get home. So we had to pay extra and nothing was ever organised we constantly had to do it ourselves. However the newspaper listed 250 tour operators to avoid... so good luck finding one thats not dodgy. My recomendation is its easy enough to do by yourself and you can easily do it at your own pace. Just get train tickets and entry tickets in advance as they sell out. If you do choose a tour operator DO NOT book your bus tickets leaving Cusco through them as they will book you the worst seats and do a runner on you because they know when you leave (at least thats what Jenny at Mapi Trek Peru does to 8 people every day)
TIP: Lonely Planet says Aguas Calientes is expensive for food.. you can get decent meals or set menus for 10 solas?? Just avoid the sharks by the bus stop... if they offer free stuff it means they'll add it on as a 'service charge'
Good luck on your travels.