Si quieres... is the right way to say it. Always. Meaning
If you want...
Si quieras... simply does not make sense (at least in Spain). It is true that it can be understood (as @EmilioGort mentions) like you think that the person who you are talking to probably does not want what you are asking. But this is some kind of interpretation. It could be something like
Maybe, if you want..., but again, it shouldn't be a valid sentence.
Siquiera... is a different word that means 'even', but honestly, is barely used, not like
Ni siquiera..., which means 'not even', and is used a lot.
¿Cómo quieres... most of the times will be used in a question, where
cómo would mean
how (and by the way it should be
cómo with the acute diacritic mark in the first
o). An example:
¿Cómo quieres que salga de aquí?, meaning
How do you expect me to get out of here?(Note that the second verb is indeed in its subjunctive form).
There is a way to use this in a different case than the question. For example:
Como quieres salir, deberías ponerte un abrigo, which means
Since you want to go out, you should wear a coat. (Weird, but totally valid)
Como quieras... may be translated as
as you want. So in this case,
como would mean
how. (Of course) you could use it in a different way, like
Voy a comprarte este juguete, como quieras luego el otro, me enfadaré contigo, meaning,
I am going to buy this toy for you, if you want the other later, I will be mad at you. It is something more like a "threat" (But ok, maybe this is a bit complicated situation :P )
Hope it helps :)
When I say that something is not used or used in a way, in general, I mean in Spain. There are some things that are pretty generic, but some of them are from a specific region. ;-)