I usually just sow straight into seed compost, with added vermiculite, in the tray, cover with the lid, then pot on when the first true leaves appear, into 3 -4 inch pots with fine compost. Only with big seeds like sweet peas, beans etc. do I sow into pots, then still use the 3 inch pots or a tray of plug shapes to get them started. When they are a decent size I pot on to 6 inch pots, harden off after a month or two, depending upon the weather, then put outside either into the ground or a final pot if that is their destination - as it often is here. I rarely sow much before April or May here, got caught too often with over enthusiasm when the sun shone in March or April, though I might start a few things then like tomatoes indoors in the cool greenhouse, but never earlier than that.