One technique for this that I like: "Talk to somebody. Talk to anybody." (Ideally somebody who cares about you.)
You don't have to talk about your suicidal crisis. You don't even have to talk about mental health at all.
Topic of discussion doesn't matter. It could be as mundane as the weather or a recent movie release or what they're cooking for dinner. This obviously isn't going to solve whatever issues you've got going on, but the goal here is to buy yourself time to ride out the darkness while also interrupting your downward spiral and lessening its intensity, and redirecting your attention away from the cycle of catastrophic thinking that often fuels a suicidal crisis.
One of the ways this can improve the acute situation is that it counters the body's "fight or flight response" (the body's physiological response to a severe acute stressor), which will be a common factor for anybody in acute crisis. By talking to somebody you trust -- someone who cares about you -- you're activating a feeling of "social safety" which goes towards calming the body down from this physiologically stressed state, because even a mundane conversation can trigger a sense of mattering or belonging that, even if you're feeling so far down, still allows something inside you to recognize you're not entirely alone. Even something as simple as tone of voice or body language can help reduce the body's stress response.
Also, this steps you out of isolation and forces you outside of your own mind. This is especially so for suicidal inclinations that are focused on past failures or future hopelessness, where a conversation with a fellow human being basically forces you "into the present moment" which, too, acts as a disruptor to that spiral of suicidality.
This is of course not curative. It's only meant as a temporary relief or as a means to buy yourself time or as a way to get yourself through the moment. Or the day. Or the night. After whatever time passes, of course you could become vulnerable again to spiralling.
So, if/when you find some temporary relief, those are the periods of time to work on crisis planning. If you have crisis plans already in place, then it's just a matter of picking one and running with it like you'd run through a script. For the "talk to somebody, talk to anybody" technique, maybe your plan has details with a list of people and contact information to reach out to, as well as small-talk topics you could talk about (recent movies, what pets have been up to, how things are going with the family, etc.). Maybe record notes along with each person on your contact list about who would be best to contact and under what circumstances or topics. You could also record how each individual person has helped you in the past (even if they've only unknowingly helped you), and such notes could act as evidence to remind yourself during future moments of crisis that, yes, it is indeed worthwhile contacting somebody in a moment like this.
Then, if you get yourself to a point where you're spending less time and energy battling periods of crisis, then perhaps you can start allocating that time and energy to other techniques that go more towards progressive symptom relief over the longer-term.