I can help with the garland. You need a length of thin rope the same as your mantel, plus extra if you want it to hang down on either side, rope width 1.5cms, and a reel of binding wire to attach your foliage. You need to attach the wire to the end of the rope before you start. Take a few pieces of foliage and wind the wire around binding them to the rope turning the wire twice. Repeat this with subsequent bunches, overlapping them to hide the stems/wires/rope on all sides.
When you reach about 30cms before the other end, reverse the direction of the bunches, taking care to cover stems etc. This means you will have a nice finish at the other end. Secure the wire safely.
You can attach all sorts of embellishments; cones, baubles, Christmas ribbon bows, dried orange slices, bunches of cinnamon sticks etc. Please don't have anything with fresh berries as they will dry out in the warmth and drop off, you don't want the risk of the DC or cat eating them. Actually, drying out and wilting are a problem with this type of garland, you would need to make it very close to Christmas because they usually only last a week (even if no fire and no radiators close.) The only exception is if you use the blue spruce which is non-drop and dries up with it's needles still attached, you can sometimes find branches for sale at florists for a couple of quid each but one branch won't make a whole garland.
If you wanted something that you could water to keep it going, you could do a long arrangement in florists foam that would sit on the mantel. Take care to protect your mantel if it is wooden from any water damage, as even though the florists foam is usually placed in plastic trays you can still get drips escaping. A piece of polythene would be fine. You can get what you need at Hobbycraft.
Another more permanent option would be to use exclusively dried items, buy a rustic natural fibre rope, and attach lots of cones, dried seed pods, cinnamon sticks, star anise, dried orange slices, woven twiggy balls and stars etc anything natural.