It's a lengthy process but definitely worth it as without doubt it'll up the catch rate.
Firstly due to the time it takes I would make sure that you salt enough boilies to last you a season, that being solely hookbaits, not freebies... personally I find that 2 kg is easily enough to do me for a season, obviously consider if you use different baits throughout the year than a percentage of each should be salted..
1. Take out the baits from the freezer and air-dry them, when I say air-dry I mean 4 or 5 weeks so they are as hard as bullets, if you can bite one and leave an indentation then let them dry a bit longer.
2. Once air-dryed, put them back in the freezer for about 2 - 3 weeks
3. Go and have a word with your local butcher, ask him to get you a bag of curing salt it usually comes in 3kg bags and will cost you about £6.00 its the stuff they use to pre-brine hams, fish, gammons, etc before they smoke them and it is what preserves the finished product... this salt will be enough to last you about 10+ years as you can re-use it over and over and over again...
4. Take the baits back out the freezer and stick them in a bucket and completely cover the baits in the salt... it is essential there is no boilie whatsoever visible on the surface as your baits (if visible) will suffer oxidation which you don't want, also it is essential that each bait is completely covered in the salt on all surfaces, achieve this by giving them a good shake and then topping up with salt if required
5. if your bucket requires the baits within to be in very close proximity then you need to be re-shaking the bucket every couple of hours for the first 6 - 8 hours to ensure they are covered in salt throughout the period of defrosting, personally I have about lots of the large maggot pots (square ones) and in each of them I usually put about 40 baits this ensures there is a decent barrier of salt between the baits.
6. The way that it works is through a phenomenon called capillary action' the bait acts as if it is a sponge, in the same way (in principle) as when you take a joint of meat out of the oven, its advisable to let it rest for a while before carving and when you go to carve it any juices that had seeped out in the cooking process will be 'sucked' back up by the joint during the resting period..? Likewise because you eliminated all moisture from the bait by air drying it, by freezing it you cause what is called 'thermal contraction' and as there is no water within the bait to expand, the 'fibres' of the bait do so instead, this causes the bait to become 'pourous' hence it absorbs the salt.
7. Leave the baits in the salt indefinitely they cannot become 'too' salty, but I would not consider using a bait that hadn't been cured for at least a month (and then only on a 12 hour rod) after 2 months I'd be happy to leave a bait out for 24+ hours.
If once cured you cut a bait open you will see preferably with a magnifying glass) trails of salt throughout the finished bait, by leaving the baits in the salt you achieve what is called a 'saturated environment which is when the bait can absorb no further salt. you will over time learn at what point this process occurs in regard to the baits you chose to use, at this point you can if you wish take the baits out of the salt and leave them hanging in a dry environment indefinitely
Now consider that although filled with salt the baits are still dry, you can now do three things with them if you wish, you can use them as is as hookbaits, you can (from a airdried state) rehydrate sufficient for your intended session 24 hours prior to going in lake ( stress LAKE) water plus any liquid flavorings you deem required, this will of course as you are replacing the lost moisture will 'dilute' the salt content as the moisture your offering is absorbed, none the less still a very effective bait.
The better option (in my 'time-judged' opinion) is once the baits are at full saturation, remove them from the salt, bag them up into individual 'portions' so if you would generally use 6 hookbaits on a day fish bag them as 8's so you have spares to replace after you catch fish (I find plastic coin money bags from the bank ideal) and then stick them back in the freezer, then the night before you go fishing remove them and stick them in a glug, due to the density of the glug it will absorb into the boilie but lock the salt in the boilie too, plus it obviously coats the outside.
When not to use them: That's easy, I have used them all year round to great effect so in my opinion there isn't a time when you can not use them.
When to use them: As said all year round, but there are times of the year when they are superbly effective, the first being spring when the fish are waking up. The second being summer after a hot period when they are not interested in feeding and are just cruising the middle and upper layers which is when the inimitable zig-rig puts in an appearance. the third is as winter approaches and they are finishing their 'munch-month' that's when you'll piss people on the lake off as your catching still and they're blanking as 'munch-month' has finished...lol. the forth is in the depths of winter when we've had some consistent pressure and they're moving about quite slowly and people cant get them to take a bait...
With you and I we achieve all our essential vitamins and minerals usually from within our daily food, when we are feeling a bit 'run-down' we may consider a multi-vitamin as a perk-up.
Carp are not that intelligent, with them it is purely an instinct that tells them they are in need of minerals and vitamins so off they go in search of minerals and vitamins... vitamins are usually achieved by HNV baits, but for the minerals they 'generally' have to look elsewhere, this is the untapped side of carping that we salt bait users are tapping into, inevitably your bait will be leaking flavoids and amino signals but also 'salt' which is a mineral, now the fish hones in on the mineral signal, picks up the bait to taste it and see if thats the mineral he want and bingo... smile for the camera...
I assume you have seen a lake that is drained, have you noticed that there are holes around the lake that would not have been there when it was filled in the first place..? ever wondered what caused them..? inevitably the fish did, and yea some will have been bloodworm beds and the like that the fish have taken, but if you go out and grab a fistful of silt from the bottom of a hole I'll bet you'd find holes where the silt smelt briney..? food for thought...?
SPRING: they are just waking up after the winter snooze and they need to get their amino's and their minerals back up to par, hence why I say when they are waking is a good time to have a few.
SUMMER HEAT: like you and I when it's too hot the last thing we want is dinner, as such our mineral levels drop, likewise in fish, but unlike us who will not eat and then when we get a headache we whinge "we caught the sun"... rubbish, we neglected to maintain our salt and mineral levels and have suffered the consequences, but in carp, they are not so stubborn, when instinct says "Go get salt" thats what they do...
PRE WINTER SHUTDOWN: just like most wild creatures, carp will gorge themselves to see themselves through the cold winter ahead (munch-month) they will make sure that they have enough salts and minerals to ensure they will get by... in the same way that Bears gorge on salmon which have a high protein count, they also lick rocks for the minerals thay exude...
WINTER EVEN PRESSURE: when they go a roaming in the winter when things are calm and the pressure is stable, why is it so hard to catch them...? I believe that it is because they are not looking for protein and foods, they got what they needed in munch-month, I believe that they are searching out minerals as these are what will dissipate from their body's far quicker than a protein will...? Food for thought..?