In an ideal world, all wildflower seeds would be sown into bare soil. That's because any existing grass will compete for the light that the seeds need to grow. It is still possible, it just needs some effort!
Mow the existing lawn as short as you can and then scarify to break up the thatch of the grass. Then cut the grass again, as short as your mower will go. Repeat these steps if you need to. The idea is to expose as much soil as possible, so you see roughly 50% bare earth and 50% grass.
The wildflower seeds grow slowly, and they need to establish before the faster growing grass overwhelms them, so the more bare soil there is the better!
Mix your seeds with a small amount of dry sand to make it easy to scatter them evenly; this is particularly useful with 100% Wildflower Seeds, as it can be difficult to sow at such a low seed rate. Use bamboo canes to mark out 1 metre squares to help you sow at the recommended rate and follow the g per sq m measurement on your seed packet. Typically it will be 2g per 1m² for a 100% wildflower seed mix and 4g per 1m² for an 80/20 meadow mix.
If it's feasible, lightly press the seeds into the soil using the back of your rake, or gently tread them in. For larger areas you can use a roller. This will ensure good germination, but take care not to bury them too deeply.
Gently water the area with a fine spray hose or with a watering can if there is no rain forecast within the next couple of days.
Early meadow management
Be patient, as wildflowers can take some time to germinate and establish. It may take a few months.
Mow to approximately 5cm (2”) after 6-8 weeks and repeat every 6-8 weeks during the first summer, being sure to remove any perennial weeds. Doing this will give the seedings the best chance of establishing, as it will stop the fast-growing grasses overwhelming the wildflowers. Try to delay the first cut until any annuals, such as poppies, have flowered.
Remove cuttings to allow developing seedlings to gain enough light. Cuttings can be left for a few days later on in the season to take advantage of seed drop, but be sure to remove the cuttings after a few days. Leaving cuttings to rot will improve the soil, which will allow undesirable weeds to become more established.
Once the wildflowers have started growing, reduce watering frequency but continue to provide water during dry periods until they are well-established.
Reduce mowing frequency to allow the wildflowers to bloom and set seeds.
Tips
Choose a time to plant when rain is expected, or plan to water regularly until the seeds germinate and establish.
Avoid using fertilizers that are high in nitrogen, as this can encourage grass growth and compete with the wildflowers.
Remember that success may vary depending on local conditions, and some experimentation may be needed.