Although the sequence of steps involved in the assembly of the PIC can vary, in general, they follow step 1, binding to the promoter.
# The TATA-binding protein (TBP, a subunit of TFIID), TBPL1, or TBPL2 can bind the promoter or TATA box. Most genes lack a TATA box and use an initiator element (Inr) or downstream core promoter instead. Nevertheless, TBP is always involved and is forced to bind without sequence specificity. TAFs from TFIID can also be involved when the TATA box is absent.  A TFIID TAF will bind sequence specifically, and force the TBP to bind non-sequence specifically, bringing the remaining portions of TFIID to the promoter.
# TFIIA interacts with the TBP subunit of TFIID and aids in the binding of TBP to TATA-box containing promoter DNA.  Although TFIIA does not recognize DNA itself, its interactions with TBP allow it to stabilize and facilitate formation of the PIC.
# The N-terminal domain of TFIIB brings the DNA into proper position for entry into the active site of RNA polymerase II. TFIIB binds partially sequence specifically, with some preference for BRE. The TFIID-TFIIA-TFIIB (DAB)-promoter complex subsequently recruits RNA polymerase II and TFIIF.
# TFIIF (two subunits, RAP30 and RAP74, showing some similarity to bacterial sigma factors) and Pol II  enter the complex together. TFIIF helps to speed up the polymerization process.
# TFIIE joins the growing complex and recruits TFIIH. TFIIE may be involved in DNA melting at the promoter: it contains a zinc ribbon motif that can bind single-stranded DNA. TFIIE helps to open and close the Pol II’s Jaw-like structure, which enables movement down the DNA strand.
# DNA may be wrapped one complete turn around the preinitiation complex and it is TFIIF  that helps keep this tight wrapping. In the process, the torsional strain on the DNA may aid in DNA melting at the promoter, forming the transcription bubble.
# TFIIH enters the complex. TFIIH is a large protein complex that contains among others the CDK7/cyclin H kinase complex and a DNA helicase. TFIIH has three functions: It binds specifically to the template strand to ensure that the correct strand of DNA is transcribed and melts or unwinds the DNA (ATP-dependent) to separate the two strands using its helicase activity. It has a kinase activity that phosphorylates the C-terminal domain (CTD) of Pol II at the amino acid serine. This switches the RNA polymerase to start producing RNA. Finally it is essential for Nucleotide Excision Repair (NER) of damaged DNA. TFIIH and TFIIE strongly interact with one another. TFIIE affects TFIIH's catalytic activity. Without TFIIE, TFIIH will not unwind the promoter.
# TFIIH helps create the transcription bubble and may be required for transcription if the DNA template is not already denatured or if it is supercoiled.
# Mediator then encases all the transcription factors and Pol II. It interacts with enhancers, areas very far away (upstream or downstream) that help regulate transcription.
The formation of the preinitiation complex (PIC) is analogous to the mechanism seen in bacterial initiation.  In bacteria, the sigma factor recognizes and binds to the promoter sequence. In eukaryotes, the transcription factors perform this role.