If you’re painting a whole room, move all the furniture out if possible. Protect the floor with a drop sheet. Mix the paint thoroughly. Use a paint stirrer and stir from the bottom upwards until the colour is even.
Start with the ceiling, cutting in the edges (leaving a fine edge) around the scotia (the mouldings which cover the line where the wall and ceiling join) with a brush. Fill in the main area with a roller, working from the darkest corner of the room towards the light.
Work in narrow strips so you can clearly see where you have stopped and started. Paint around ceiling light fittings, then complete the rest of that band.
Next, the walls. Start in the corner, cutting into wall edges, window and door frames.
Fill the main areas using a roller, working along the wall evenly in an up and down motion.