A yoga block can bring a support surface closer to you. Use it in the position your instructor demonstrates, at a height that lets you remain comfortable and controlled. Its purpose is not to force your body into the deepest version of a pose.
Blocks are ordinary practice tools, not a sign that you are doing yoga badly. The useful question is what the block is supporting in this particular movement and whether the setup works for you today.
Find out what the block is meant to do
A teacher may use a block to support a hand, raise a sitting surface or modify another position. Those uses are not interchangeable. Ask where it belongs and what part of the movement should feel different, especially if you are following a recording without individual feedback.
Manduka's prop guide describes blocks in different shapes and materials for support. That does not make every possible placement suitable for every person. Learn a specific use rather than borrowing a position from an unrelated photograph.
Choose the height before putting weight on it
A rectangular block can offer different heights depending on which face rests on the floor. Set it down fully and check that it does not wobble. Use the height recommended for the movement and change it before settling into the position, not while trying to balance on it.
If you need two blocks, have both within reach before starting. Do not stack an improvised tower to reproduce an image. Ask for another modification when the available equipment does not give you a stable, comfortable setup.
Check the surface and the block
Place it on a stable surface in the clear practice area. Look for damaged edges or a surface that makes it difficult to keep contact. Follow the manufacturer's care instructions for that material, and allow a cleaned block to dry as directed.
Foam and cork can feel different under a hand or body part. Choose based on the intended use and what you can control, not a belief that a firmer or heavier block automatically makes the practice more advanced.
Keep the movement comfortable
The National Center for Complementary and Integrative Health advises learning yoga with appropriate instruction and discussing relevant health conditions. Some people need modified positions, including during pregnancy or with certain joint or balance problems.
Tell the instructor if the support is uncomfortable or the position causes pain. Do not move farther simply because the block makes the floor easier to reach. The goal remains an appropriate version of the movement, not a bigger range at any cost.
Remember the setup that helped
Keep spare props outside the area where you will step or turn. Having support available should not leave a loose block where your foot needs to land during the next transition.
After class, note which face of the block you used and where the teacher placed it. A short note can make the next practice easier to repeat. Keep asking about unfamiliar uses. One well-understood modification is more useful than a collection of elaborate prop arrangements you cannot set up confidently.