This is the easiest way to make sloe gin in my limited experience.
Pick sloes. These look a bit like blueberries but they grow on blackthorn trees often amongst bramble in hedgerows. Google images so that you know what you're looking for. The important thing is to recognise the blackthorn leaf (narrow and about two inches long) as well as the berry. They appear in August/September and stay on the trees until October. If you don't plan to freeze them then you should wait until after the first frost.
The sloe is part of the plum family and is like a smaller, more bitter version of the damson. They taste vile.
In order to use one litre of gin, you will need 425g of sloes. If you use fresh sloes then they will need washing and checking for creepy crawlies. You'll then need to spike each berry with a pin to pierce the skin. I wash, check for bugs and then freeze the berries which dispenses with the need to prick the skin. You can probably use them frozen, but I defrost before using. I open freeze and then, once frozen, scoop into bags for most efficient use of freezer space.
You will need two receptacles: one for the "brewing" process and one for the bottling. Used gin bottles are fine for both of these but Kilner jars are also fine for the brewing process. My preference is the "alcoholics special" 1 1/2 litre gin bottles for the brewing process (Sainsbury sell gin in 1 1/2 litre bottles and I'm sure they're freely available everywhere) and Kilner or Kilner-style bottles (IKEA do a range as do Wilkinsons) for the bottling procedure.
All vessels must be sterilised (allegedly). I do this by cleaning them and rinsing thoroughly in hot water before bunging glass/ceramic bits in the oven at a temperature of about 80-90 degrees Celsius for about 20 mins. Wait until hand warm before handling. All rubber seals/metal lids get boiled in water for about ten minutes. If you're using "fresh" gin bottles then I reckon you can skip the sterilisation. I'm still unconvinced it's absolutely necessary as alcohol kills bacteria (doesn't it?) but I'm too scared to risk it.
If using a litre of gin then pop 225g caster sugar into a 1.5l bottle or Kilner jar that you have sterilised. Follow this with 425g of sloes. If using a bottle then you just pop them in one by one. Finish off with the gin.
Lid on and shake it all about. Store in dark room (I don't know why it needs to be a dark room but I use kitchen cupboards or the cupboard under the stairs.)
For the first week you need to invert/shake daily to try and dissolve the sugar. After the sugar is dissolved you need do this once a week for a month. After this you should have a beautifully ruby red syrupy goo that just needs to mature. The longer you leave it the better. Generally if you make it in early October it should be drinkable for Christmas.
Prior to drinking you need to bottle it. You'll need muslin (the type used for preserving not the type you use on babies), a funnel and a bottle.
It's at this point you'll appreciate my advice regarding the use of the alcoholics special 1 1/2 litre gin bottles.
Arrange funnel into the top of the sterilised bottle. Arrange muslin such the all liquid going into the funnel has to pass through the muslin. I do this by stretching the muslin over the top of the funnel and holding in place with a hi-tech elastic band. I then make the muslin a bit "baggy" so that stray sloes don't bounce off the muslin.
Pour from brewing container into muslin adorned funnel. Be careful not to over-pour. The advantage of brewing in a bottle is that pouring process is quite un-messy. Take it from me that pouring from a Kilner jar will break your heart as the ruby red elixir will spill onto your work surface without any opportunity for retrieval.
It will keep for a really long time, not that I'd know...
It tastes like cough syrup remembered through rose-tinted tastebuds (not sure that works linguistically), is good to keep and lovely to give.
I would recommend that when sloes are on the trees you go mad, pick loads and make as much as you can.