An essential element to contract or agreement is the "meeting of minds" ("acuerdo de voluntades"), so what is being established here is that the parties are aware they are entering into a formal bargain governed by those contractual clauses.

https://en.wikipedia.org/wiki/Meeting_of_the_minds

"Based on the foregoing [representations], the parties express their intent to enter into this agreement, subject to the following clauses..."

https://assets.publishing.service.gov.uk/.../Heads-of-Terms-...
This document sets out the principal terms and conditions on and subject to which the following parties (the Parties) are willing to enter into a Collaboration ...

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Note added at 4 hrs (2019-05-02 16:08:09 GMT)
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p.s.
you don't need to repeat parties, i.e.

since/as the parties are willing to enter into this agreement/contract, they submit to...

*Pursuant to the foregoing (recital(s)*, desirous as they are.....

Don't forget the desire(s), as a time-hono(ured )English legal drafting technique!

We will leave for now any conclude = end or complete vs. (AmE) closing of contract issue for 'celebrar.

Medium CL - confidence level as linguistically, rather than legalistically, not a straight match, in deference to the other contractual intent(ion) etc. points.

NB City of London/UK lawyers dislike a translation as a (binding) Contract, perferring - 'as they do' non-committally on their clients' behalf - the term of (a preliminary) Agreement > see Jean's answer, for reasons that need not be canvassed here.

This is basically a boilerplate phrase that probably comes after the recitals and before the contract provisions. The classic (albeit highly archaic and superfluously formulaic) would read something akin to "NOW, THEREFORE, in consideration of the premises and mutual promises herein contained and for other good and valuable consideration, the parties agree as follows:"
You could probably also just simply write "The parties therefore, intending to enter into this agreement, hereby agree as follows:"

Whether you really want to translate "cláusulas" is up to you; it would also depend on the purpose of the translation.

For all practical legal purposes, a simple "The parties therefore agree as follows" would probably suffice, as Ken Adams suggests (see link below).

The part about "siendo su voluntad" would typically be expressed in the recitals (such as in the link provided by Adrian in his answer) just above the "Now therefore" phrase.

Good luck!

Con base en lo anterior y siendo su voluntad celebrar el presente contrato, las partes se someten a las...

In view of the above, and considering that the parties desire to enter into this contract, they submit to the...