You should begin sowing in May using 3 inch pots filled with the bottom inch or so filled with in a good quality John Innes seed compost, but if you have a tendency to over-water then you may wish to add a handful of horticultural grit to the mix to improve the drainage. Take one seed and place it either on its side, or with the pointed end down, then fill the pot to within 1 inch of the top with more of the compost mix.
Water in and then place your pots onto a warm, sunny windowsill. The seedlings should emerge within 7 – 10 days. Now these new plants will grow pretty fast from this point and so it’s important that they don’t dry out – they will probably need watering every 2-3 days, but make sure that they don’t become water-logged otherwise fungal rots could appear. Even at this stage you can consider feeding ½ the recommended dosage of an appropriate liquid fertiliser.
After about four weeks on from sowing, your squash seedlings should be ready to go outside, but they will need to be hardened off for a week or two before being left in their permanent position. You have two choices at this point. So long as the threat of late frosts are over you can either plant directly into the ground or – if you are short of space – plant into large containers, but when it comes to butternut squash, the larger, the better!