A writing session does not need to begin with an important subject. A particular sound, object or brief encounter can provide enough material to practise description. Choose something you can observe closely rather than something you feel obliged to make impressive.
Write the concrete details first. Where does the light fall, what changes and which features distinguish this moment from a general idea of it? Leave interpretation for a second pass.
Next, select the details that serve the piece you want to write. Accuracy does not require including everything. A reader needs a coherent point of attention rather than an inventory of every object in the room.
Save the draft and return later with a simple question about what interests you. Some observations will become longer pieces and others will remain exercises. A practice can be useful even when it produces no immediate material for publication.