UK refusal of entry impact on entering the Schengen zone

Regardless of your nationality, you can apply in advance for a Schengen Short-stay visa from within your home country. If your nationality is not on the visa-national list and you would not normally need a visa in advance, you can still apply.

Doing this has the advantage of maximising the chances of successful entry (all other things being equal) and avoids wasted airfares and the distress of removal from port.

You can do the same thing for a UK Standard Visitor Visa. People can apply for entry clearance even though nationals of that country do not ordinarily need one. Once again, the important caveat for successful entry is all other things being equal.

You did not provide your nationality or location so more specific information about how to apply cannot be given, but the generic steps are...

1. Plot your itinerary and contact the Embassy of the country you wish to visit
2. Download and fill out the form
3. Collect your evidence
4. Report to the VFS for biometric enrolment and visa submission
5. Await results

To summarise...

How can I be assured I will be admitted to Schengen countries?

There is no method that absolutely guarantees admission to the Schengen zone. However, applying for a visa in advance maximises the chances of a successful landing and minimises the chances of removal from port (all other things being equal).

You cannot be assured of admission. All countries in Western Europe, like nearly every place in the world, reserve the right to refuse entry to aliens for a variety of reasons, and their immigration officers have the power to do this even for those who have received a visa.

Note in the Schengen Borders Code that possession of a valid visa is only one of several conditions of entry (Article 6). Another condition is that "they are not persons for whom an alert has been issued in the SIS for the purposes of refusing entry." If such an alert has been issued for you, then you are out of luck.