Subject: How do you successfully cross two different species of plants?

I previously asked about making a new breed of plant using molecular biology. The response was that 
since I don't have access to a molecular biology lab:

"Your best bet in terms of creating a new species might be to try and create a polyploid
species by crossing two parent species that were themselves polyploid, but
with different ploidy numbers."

Does this mean two different species that were just different ploidy numbers? And how would I go 
about crossing two different species of plants? "The standard definition of a species is one that has 
evolved to the extent that it can't reproduce successfully with other types of organism." Some closely 
related species such as horses and donkeys or lions and tigers can breed but the offspring supposedly 
is sterile.. But I read that some female "ligers" (Male Lion and Tigress mix) are fertile. I'd like some 
clarification on that.. But my main question is still how would I be able to successfully cross two 
different species of plants to create an offspring that would be considered a new species?

Re: How do you successfully cross two different species of plants?