Digging out for the base
Dig out the topsoil and save for elsewhere in the garden. How far you dig down below the topsoil will depend on the patio.
For light use, the slabs can be laid on a compacted 50mm bed of sand. More stable, and recommended for most patios, is a 50mm semi-dry mix of sand and cement.
Areas of heavy use should have a compacted 75mm layer of hardcore below the sand/cement mix. For vehicle use, the hardcore bed should be 150mm. Hardcore is broken brick and rubble, which is laid over the ground and compacted to provide a firm stable base. The area of the patio multiplied by the thickness of hardcore needed will tell you the volume of hardcore required, but remember to allow a bit extra as it will be compacted.
It is worth hiring a plate vibrator for the preparation stage to ensure that the base is very solid. Once you have prepared the base, begin to lay the flagstones in one corner.
Laying the slabs
For light use, trowel five generous dabs of mortar on to the base and lay the first slab in place. Tap the surface of the slab with a rubber mallet or with the end of a club hammer and check with a spirit level in both directions remembering to allow for the fall. This is where the string between pegs really helps since you can use it as a guide.
For heavier use, the slabs should be laid on a continuous bed of mortar, 50mm thick, rather than dabs. This way, the slab is fully supported and will be less likely to crack.
You can also lay paving directly onto the sand. This will need to be carefully levelled to support the slabs fully. By putting battens at intervals in the sand, with their tops at the appropriate level, use a straight edge to level the sand between them by drawing it along the battens. Remember to remove the battens and back fill the groove with sand before paving over as the battens will rot if left in place..
Lay the next slab. Some paving can be butted together but there will usually be a 5-10mm joint. Cut pieces of hardboard to use as small spacers and wedge them between the paving to keep this gap even.
Continue to work across the patio area, regularly checking the level. It's best to work across along the house wall first, then down one side using the guide string before filling in the middle, working diagonally.
Cutting the paving stones
It is usually easiest to leave the slabs which need to be cut until the bulk area has been laid.
Now, fill in gaps with cut slabs. You can use a grinder or bolster chisel to score all around the slab.
Angle grinders can be hired. Always read the operating instructions and if you are unfamiliar with the equipment, ask for a demonstration. As with all cutting work, wear safety goggles. Protect your hands with work gloves and with noisy equipment, wear ear defenders.
Once scored, raise the smaller section on a thin batten and tap the main part of the slab along the cut line with the handle of a bolster hammer.
Finish off by forcing a fairly dry mortar mix into the gaps with a trowel and brushing off the excess before it dries.