I wish to perform USER cloning to assemble multiple fragments. But I have no prior experience in doing it. I want to know what are the criteria for designing the USER primer to ensure that the PCR and the cloning works.
I suggest you to use the primer blast tools: http://www.ncbi.nlm.nih.gov/tools/primer-blast/. Some of the factors you need to consider: First you should have Gene sequence, and then PCR product size (I use between 70-100), Primer melting temperature (Tm), Primer length, GC%, position of GC and so on. For advanced primer designing please find attached a document that could help you.