Garlic I used to follow the "plant on the shortest day and harvest on the longest" maxim as it worked. If you plant them sometime near dec 21 ( the shortest day), you can harvest them on the longest day ( June 21 ish) Any time after the end of October will help the plant to establish a root structure which will help it to grow in spring. However, don't plant it in your planned (next years) onion bed as it may encourage the growth of whiterot which can decimate an entire crop of onions and leave the soil unonionable for years. For this reason ( whiterot is endemic on allotments, I think it is because the old hands did not practice crop rotation) I have given up on the traditional timings and I just put them in at the same time as the shallots: ie end of February beginning of March.
Method: Choose a warmish day
Prepare the soil.ie remove weeds and rake it a bit
Split the garlic head into cloves
discard any loose stuff
make a hole with a stick so top of clove will be at least 2" below surface and 6-9" apart (the bigger the gap the more chance of larger crop)
insert clove pointed end upwards
roots grow from bottom (flattened end), foliage from top (pointed end)
you may need to net against magpies/other birds if they are a problem because they like pulling at things and tend to chop them off.