The hardening off process should take about two weeks before planting outside. For tender plants (like tomatoes, runner beans, sweetcorn, geraniums, salvias, buzzy lizzies, begonias and petunias) start the process around the last frost date. Some plants can withstand a degree or two of frost (alyssum and antirrihnums), these can be hardened off a month before the last frost date and planted out a week or so before the last frost date.

For the first week, ventilate the cloche only on warm days, and always close the cloche in the evening and night. During the second week, gradually increase the ventilation period, leaving it off in the evening where no frost is threatened. 

No Cloche Available?
Examine the garden on a frosty morning to see which areas are relatively free from frost - choose these areas for your plants. Any protective barrier you can provide against the wind chilling the plants will greatly help. During the first week, place the plants outside during the warmest part of the day (a few hours at first) bringing them back indoors / under cover after this time.