Mahasi Vipassana instructions
Vipassana Meditation Instruction by Sayadaw U Zatila
Generic (posture, flexibility, pain etc.)
Dealing with Pain in Sitting Meditation by Neal Golub (He shows various sitting postures - the desired one is the “Burmese position”)
Stretches for the Inflexible by Nico/PsychTruth
Yoga For Flexibility by Adriene
Note: The last two videos are not related to meditation. However, the instructors present two very useful exercise routines for meditators who desire to improve their flexibility.