One of our lads left the club to go to a larger club where he was coaching a youth side. They had a flicx matt and he wasn't impressed by it at all.
The best thing to do is dig out the soil from the garden, should be around 200-300mm deep. Timber edging to the perimeter fixed to timber pegs. Use type 1X or type 3 sub base for the base, 150-200mm deep and compact it well with a vibrating plate. You will then want to lay approx 40mm of a 2-6mm graded gravel up to the level of the timber edgings. This should also be well compacted, you could screed this layer off the edgings (provided you have put the edgings in level) the mat can then be laid over the timber edgings and fixed onto them to keep it in place.
I'm sure that the depth/material used for the top surface (under the mat) makes differences in the playability of the wicket. I'll see if i've got any files from previous jobs on my computer