The accuracy of these positions will carry over to your pre-pointe strengths, your adagio, to......everything you do in a ballet class!
Learning correct body placement and fine foot work in these exercises will also help prevent injury.
For a la seconde, the foot leaves fifth or first, the sole pressing into the floor, the metatarsals pressing as you extend the arch, and lastly, you lengthen the toes.
Hopefully nothing else has happened in the body or to the standing leg. You aim the tendu to the spot furthest to the side, where you can still hold your turnout in both legs. For most dancers, this is not straight to the side.
It doesn't matter.
Holding the turnout of the supporting leg and the placement of the body facing square to the front matters.
You need this stable position for developpe, turns in a la seconde, and jumps in or going through a la seconde postion (fouette saute, grand jete en tournant).
Closing the tendu, press the toes down, then relax the metatarsal joints. Press the ball of foot and sole of foot into the floor, creating resistance.
Make sure the whole foot is on the floor, so you can smoothly resume weight onto it. All the way into first or fifth behind, aim for the foot placement that allows maximum turnout of both legs from the hips, no wiggling, and minimal hip change.
A la arriere, behind, press down into the foot, changing the weight to the supporting leg. Lead out with the toes first, lengthening down the back of the leg, and continue to apply pressure in the sole of the foot as the arch stretches and then the toes lengthen.
At some point, your working hip will open from a square position, but the turnout of the supporting leg should not change, and your body from the waist up should be square to the front.
Also, you have to keep feeling length down through the leg. The leg must be extended all the way out, before the foot fully points, or you will force your torso to scrunch at the waist instead of staying long.
It's like a tug of war to lengthen the leg, and keep the body pulled up tall. Also that feeling prevents you from shifting the weight back off the supporting foot.
You should be able to lift your hand from the barre any time, and be tall on your standing leg.
Closing from the back is a gradual change from the heel leading back in, pressure on the sole of the foot, and bringing the toes forward again to where you can stand on your whole foot.
Also the working hip comes square again, smoothly, as the toes drop, the arch presses down, and the weight goes on to the foot.
Massage your feet with a golf ball or small hard rubber ball. Ice if your feet ache, and massage your feet when you're sitting watching a movie, or studying. Strengthen, stretch, and then relax too.
Learn basic ballet positions from The Perfect Pointe Book. It is a comprehensive manual which explains use of the foot muscles, and basic ballet technique.
Your tendus will strengthen faster if you add these exercises to your daily routines.
Tendu devant (to the front) is covered in detail here.