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As Kahis said, many lay tiny eggs on the host's larval foodplant and hope that the larvae injest them. Others lay first instar larvae that wait on the host's larval foodplant and grab a larva as it goes past.

As far as I know, no tachinid actually has an 'ovipositor' but many species have 'piercers'. The distinction is that an ovipositor is rather like a chitinous hypodermic needle that pierces the host integument and through which the egg passes into the host's body. But a piercer is a sharp, solid projection made from one of the sternites (I think), which is used to stab the host and create a hole into which the fly can introduce an egg, using its soft egg-laying tube. :)