To find a horse you want to back to lose (lay) you need to consider the same factors you would for picking winners. The difference is that you when looking for winners you are looking for positives, e.g. a trainer in form, a horse suited to the course and distance, in looking for losers you are looking for negatives. The more negatives the better, in particular, the five key factors given in the picking winners section.
It is not however simply a case of finding horses with negative factors you must also look at the other runners in the race and find two or more horses that have more factors in their favour. There is no point in laying a horse if no other horse in the field is good enough to beat it. If there is only one horse in the field you feel is capable of beating your selection then it may be more profitable just to back that horse rather than laying your original selection. It is the case in some lower quality races that no horses have all the key factors and the horse you have selected to lay has less negative factors than the others which would put an element of doubt into any selection. It is safer therefore to have no bet if this is the case. Remember, there will always be another race so only bet when you are sure.
The odds are the most important factor to consider when laying. You have to remember that you are offering the odds and so your liability (the amount you lose if the selection wins) is your stake multiplied by the odds offered. It may seem a horse priced at 50/1 to lay is a certainty to lose and therefore easy money but if you lay that horse for £10 and it wins you would have to pay out 50 X £10 = £500 and horses priced at 50/1 do win sometimes.
On the opposite side of the scale, laying favourites at odds on may seem attractive from a profit point of view because your liability is low but you have to realise that more odds on shots win than lose. The reason they are odds on is usually because they are the best horse in the field.
The key to laying horses is in the value aspect as explained in the concept of value section. If you feel that a horse's odds are lower than it's chances of winning then that is the time to lay.