Suspense is distinctive to other types of writing
Writing suspense is very different from writing other genres. It has features that do not come naturally to most writers. For instance, Writer's Digest defines the difference between suspense/thriller, mystery, and horror as:
"In a mystery you might find out that a person was beheaded. This occurs before the narrative begins, so the focus of the story is on solving the crime. If you’re writing a horror story, you’ll show the beheading itself—in all of its gory detail. If you’re writing suspense, the characters in the story will find out that someone is going to be beheaded, and they must find a way to stop it."
This means, rather than shocking the reader or keeping secrets from them, you put your cards on the table, tell them the world is in the balance and make it likely it will go down the drain.
One good example of suspense done in this way is the James Bond series. We know from the start or very early on the villain will try to end the world and only Bond can save it...
Use your characters
You use your characters to create suspense by making the reader care for them and then put them in danger.
You make the antagonist into a supervillain. A super character with super abilities so we start wondering if even Bond will be able to stop them. (Ok, when I say super, I don't mean batman or superman, I mean a super character as in a super skilled, smart, determined character...)
Of course, such an antagonist also requires a skilled and impressive protagonist.
In fact, if you want to make the reader care for your characters you need to spend serious time making all of them great.
Creating suspense is all about promising the reader all hell will break loose.
In fact, to create great suspense, spend more time promising violence and destruction than actually doling it out. And make big promises of physical danger as early as possible in the text.
Of course, you need to keep every single one of your promises (unless the protagonist can stop the antagonist, but they should only barely be able to do that).
If you don't keep your promises, you'll burst the suspenseful bubble...
But the mayhem does not have to start in chapter 2, only you need to convince the reader that before the end it will come.
Promises can be direct or they can be made using foreshadowing.
Another great tool for making promises is the cliffhanger.
Show the reader what will happen
As mentioned above, suspense is created by showing the reader all the risks and horrors that lie ahead, rather than keeping them secret or springing them on the reader for shock effect.
Put all the cards on the table and increase reader anxiety by showing exactly what will happen.
That does, of course not mean your main character must always know about the dangers. Only the reader.
This does open up the risk of making the story predictable, which requires fantastical villains and heroes that will attack and parry in a deadly dance throughout the novel always keeping the reader wondering how on earth the protagonist will counter the antagonist's attacks and then watch in astonished horror as the protagonist's best thought out plans are ruined by another brilliant riposte from the antagonist.
Use many perspectives to show all the risks. You can even use the omniscient perspective to great advantage when writing suspense.
Let the characters tell the reader about their plans and thus foreshadowing that there will be problems.
Conflicts, risks, pressure, and problems
Use time pressure and deadlines to increase pressure and conflict.
Make sure the cost of failure is as high as possible.
Sharpen the main conflict and every other conflict in the novel to be as sharp as it can possibly be.
While you should show the reader what will happen in order to create suspense, you should also try to be unpredictable.
This can be done by adding surprising twists and turns but also by never letting any plan survive the first contact with reality. If your characters don't think to develop a plan B, make them regret it, and if they do, make them fall back to plan C or D.
As the novel progresses towards the climax, increase the pressure, complicate things, and subject the characters to dilemmas and force them to choose between two evils.
To show suspense limit violence and work with the promise of violence instead.
Spend extra time polishing suspenseful scenes and spend extra words on them. Draw it out!
The perfect suspense story builds tension higher and higher until it explodes in the climactic moment.
This, however, can cause problems if the suspense doesn't last the whole way and bores the reader, or if the suspense can be kept high, it will also be too much and finally become background noise.
To solve this vary the level of suspense, for instance by dividing the conflict into steps with smaller victories and losses as the story takes each step.
Use character thoughts (internal emotion) to bring up problems and causes for worry to make sure the reader also worries.
Create an air of foreboding to increase suspense.
In semi-order of "importance":