you could try Primer 3 for selecting a wider range primers and annealing tem.

https://www.bioinformatics.nl/cgi-bin/primer3plus/primer3plus.cgi

Primer length is generally about 18-25 bp and annealing temperature depends primer used 50-55 degree cel. for more details pls see https://www.sigmaaldrich.com/technical-documents/protocols/biology/standard-pcr.html?gclid=Cj0KCQjw78yFBhCZARIsAOxgSx3ON7NXIqnrJiG7-boSoKxHctXfDa-FGMZ_33nSLgFi4PpcSOOzVqsaAjf_EALw_wcB

In general the same rules apply for overlap extension as any other PCR, so the most important thing is that the Tm of the primers you are using is close to each other.

However, the design it does depend on what you are using the overlap extension for. Are you using it to mutate or add an insertion/deletion to a region? Or are you trying to seamlessly join two regions of DNA together? You will need four primers: two in the region you are mutating/or joining together and two at the very end of the final desired fragment. You will initially use these separately (ie use one primer from the region you are mutating/or joining together and one at the very end of the fragment) to create two separate fragments that will be joined together in the second step (I usually just add the two fragment together with Taq etc but no primers for 10 cycles of a PCR, then add the primers to the reaction and put it through an additional 20 cycles or so).

The most important thing is that the two primers that are in the region you are mutating/or joining together create a region that overlaps so that the resulting fragments can be joined together in the second step. If you are doing mutagenesis that is not difficult, just make sure the primers are reverse compliments of each other (and adjust the Tm due to the mismatch). But if you are mutating or joining two fragments together, the design is more complicated. You will need design a much longer primer which has homology to both fragments created in the first setup: in this case the 3' of the primer corresponds to the initial fragment you will be amplifying, whereas the 5' of the primer will contain homology to the fragment that you will be joining to in the next step.

I would suggest this paper if you want more detail (or at least look at the figures to understand the design of the overlap):

https://www.nature.com/articles/nprot.2007.132

@Helen Bellchambers

Thank you Helen Bellchambers

The article you suggested describes the mutation,

Exactly i am joining three fragments together and i dont know What is the most suitable primer length and annealing temperature for overlap primers?

Bheem Dutt Joshi

I need Protocol overlap extension pcr not pcr.

https://www.protocols.io/view/overlap-extension-pcr-psndnde?step=1

Here, I made a schematic in the attached powerpoint slide. Basically the primers at the ends that are not being joined together should be 18-21bp (I usually go for 18bp) and the primers on the ends that are being joined together should be 27bp, of which 18bp should correspond to the fragment being amplified in the first round of PCR (only use these 18bp when calculating the Tm of the primer) and the other 9bp correspond to the other fragment. That way the resulting fragments will overlap by 18bp.

I usually aim for a Tm of 58 degrees, but it should not matter that much as long as it is within 3-4 degrees for all of the primers. You may have to make it slightly higher for all the primers if working with a GC rich region for example.