There will come a time when you will face the need to out cross. There are several reasons to out cross. Is it to improve your line? Or is it because of low fertility and hatchability? Maybe you haven't found a line that you like and instead found 2 different lines with each have different traits that you like.
There are 3 different ways of out crossing. 1- Out cross, then blend back in your line. 2- Out cross, then blend back to the out cross line ( if the out cross line is the better line) . 3- Out cross, then blend the 2 lines together.
Just remember that out crossing adds genes. Therefore the F1 generation ( chicks from the out cross mating ) will be varied. You may not even produce anything that you like. However keep the birds that come closest to your goal. Also use the best that you can find when you out cross. Do not use inferior stock, no matter what the price. Inferior stock will set you back.
The most common reason is to add "New Blood" due to a decrease in fertility and hatchability. It is also the
easiest to do. Once the bird that is being used for the out cross is obtained, ( it can be either male or female) it should be kept separate from the rest of your flock for several weeks. You don't need to bring in any illness with it. After it has been cleared for illness, mate it to your best bird of the opposite sex. Use a cage for pair mating. This is one of the few times when pair mating is required, even if you normally use the flock mating system. You want to keep that pair's eggs and chicks separate. In fact the natural method of hatching and raising chicks works well in this case. The hen can set, hatch and raise the chicks in her cage. What you want is the best male(s) to put back on your hens. (The male will be 1/2 your line. When mated back to your hens the offspring will be 3/4 your line) This method should not "set you back" much if at all. And it was done quite easily and economically as your only outlay was for the bird that was used. This method is best used if you have a good line and are pleased with what they are producing.
Out crossing to improve your line is harder and more involved than out crossing to add "New Blood". Use this method if you found a strain that you like better than what you have at present. This time instead of obtaining 1 bird you will need to obtain a trio ( 1 male 2 females) from the strain that you plan on using. After they have been cleared for illness, mate your best male to the 2 new hens. It is from this mating that you want to hatch the chicks. The new male can either be kept with your hens ( you don't need their eggs) or in a cage by himself. He will not be used until the following year. The following year, place that male with the pullet(s) that are from your male and the new hens.( The pullet(s) are 1/2 your line and 1/2 the better line. When mated to the cock, the offspring will be 1/4 your line 3/4 the better line) You should see an improvement in the chicks. If the improvement is less than expected, the process can be repeated with another trio. Not only is this method an economical way to improve your line, you are also doing an out cross (your line). If you started over with the better line the cost would be higher and you will still need to out cross sometime. As it is, you can breed for several years before you will have to deal with out crossing again.
You may want to blend 2 different lines together when both have some traits that you want to incorporate. This is the most involved, as not only do you want to mate and blend them together, but you want to keep both lines to go back to, and reblend, if needed. This form of breeding is very involved, and could take years before you start seeing your goals met. It not only requires time it also requires much space, as while you are doing it, it may be necessary to keep, (house, feed, mate, and clean ) 3 different lines going at once. That is why the best advice is to start with the strain that comes closest to your goals, and work with it. Blending is not recommended for the weak at heart, or the beginner.
To start, you will need to obtain a trio from each of the 2 strains that you will be using. Hatch and raise some chicks from each trio. These are your "pure" lines, and must be kept separate from each other. Then put the cock from one line to a hen from the other line. Hatch the eggs and raise the chicks as above, also keeping these separate from the others. These are your crosses. When setting up the next years breedings, choose the birds from each group that come closest to your goal and mate them together, keeping each group separate. What you will have to do is to keep mating each years chicks together, always choosing the ones that come closest to your goal. You may, at times feel the need to go back to one or both of the original lines to reenforce the traits that you are trying to bring out. However, the original lines will be slightly changed as you will be selecting each years breeders to your ideas and not the original breeder's.
When the blended line starts to meets your ideas, the original lines can be eliminated. You can sell them outright, or you can use them for an out cross to blend back in your "blended" line.
The blending of 2 different lines also will works when you have a line that you like, but saw another line that has some traits that you want to add to your line. Just remember that it may take years for your goals to be met.
This has been a quick summary of the different ways and reasons to out cross. Each involves time and effort. At times the reason to out cross is valid. However when first starting in the hobby of poultry, the best advise again is to start off with the line that comes closest to your goals and work with it. Also as time goes on, after choosing each seasons breeders, you will be slightly changing the original line to your ideas.