Open an A&P book and look at foot pictures that map out arteries. Every pt is a little different when it comes to exact location, but they generally have the same basic anatomy.
Pedal is just on top of the foot. Use an imaginary line from the ankle, down to between the great toe and second toe. Use that as a reference. Most will be on that line. Some will be a little lateral to that line. Make sure you feel from about one inch from the ankle, all the way down to about one inch from the toes on top of the foot.
Note that the artery aims right down between the toes. It should be higher toward the ankle, but there are pts who have the artery go all the way down.
The post tibial can be a little more difficult to find, but make sure to feel in the right location. Push your fingers into the talus bone.
If you can find a doppler, use that first. Then, see if you can palpate it. Use that to practice.
Make sure the pt doesn't have peripheral artery disease. You may not be able to feel anything. Sometimes, the only thing you can do is doppler it.
Wrap their feet in a warm blanket. This will increase circulation and cause vasodilation. Cold feet is common in the hospital.
For the popliteal artery, just go for the middle. You may have to really palpate. Manipulate the leg so you depress the fingers. The artery is tucked in between the two condyles.