When hiring new candidates you must focus on getting the right person for the job, no matter their characteristics (gender, sexuality, etc.). Not hiring someone because of these characteristics is against the law in many countries so be careful you do not to anything to go against this.

If you want to hire more women, ensure that the job vacancies are not discouraging women from applying. Check that gender neutral pronouns are used, if you are using 'he' in the job description for example, change it to they. There are tools you can use to check the language/words used within a document are neutral or not. (Like this one)

Advertise in spaces, forums, locations etc, that will enable more women to see it, and encourage them to apply.

Talk to HR about how they currently recruit, it could be that this work is already being done.
Sadly, not as many women are involved in STEM, and this means often there are less graduates and applicants that are women, so it's just about ensuring everyone is able to access the job itself.

Just stick to the talent of the candidates. Positive discrimination is still discrimination, and lead to worst result

Depending on your country, hire gender specific candidate could be illegal, and would serve no purpose as gender is irrelevent for the quality of a candidate.

If you want to a better ratio, make effort to have more female candidates. Maybe something in your job offer (company reputation, job description wording...) discourage woman to candidate. But once You've got your candidate's list, just stick to their skills.

Also, 1:1 ratio should not be the ultimate goal. Software development is a very gender-oriented field, but the problem is deeper than just your company. If there isn't a 1:1 ratio at software engineering school, how could you achieve this ratio in companies?.

Definitely do not ask to hire gender specific candidates. In most countries, it would be illegal to do this. HR would probably tell you this anyway if you even ask them. It has been mentioned in other posts here, but you can try to check if your recruitment also reaches schools that are more well known to be gender equal, but do not only target them, as this would also be most likely illegal.

Skillset wise, sure, you can try to get in some variety, but a key aspect to recruiting is the job offer, and the recruiters. And I think that this is not only a matter of HR, but of other people (you or someone else that is more "technical"). If only HR is involved in the recruitment, do not expect to have all your requirements satisfied.

While the other answers are correct, they is a minor exception that you will be able to hire candidates of a specific under-represented group. This is called "Positive Action" and applies at least in Europe (maybe other parts of the world as well). In UK specifically, the 2010 Equality Act provisions specifically for that.

The caveat is that in order to prefer the under-represented group candidate for a specific position, they have to be equally qualified and skilled as the other top candidates, so this should only be the last criterion for your selection.

I would be surprised if your HR department is not aware of the Positive Action. I think US has something similar that is called "Affirmative Action".

An 8:2 gender ratio in software development is the average worldwide. That's not just in the workplace either - it's reflective of the fact that significantly fewer women than men choose to study computer science while in education, similar to how in the West more women than men choose to go into the education or health industries.

Interestingly, the less advanced the economy of the country, the more equal the gender ratio gets.

The most you can legally do is widen the application pool to be inclusive of more women

You cannot legally rule out people of a certain gender from the application process.

You cannot legally use a person's gender against them during the hiring process.

You CAN advertise in places where female candidates are more likely to be.

You CAN hold or sponsor talks for getting more women into your line of work

My husband is a feminist hiring manager in IT. One thing he does is to run his job advertisement text through a tool like Textio, to help remove biased language. Also remember when interviewing female candidates that in any culture, including Japan, women are socialized to be different from men, and in IT interviews, this often comes off as a wrong fit.

Women's voices are higher-pitched (which often comes across as an inability to lead or be assertive). Their hand shakes will often be gentler (a false sign of a lack of confidence). Also, research shows that men in a job interview are judged by what the employer believes they can achieve, while women are judged only on what they've already done, making it difficult for women to advance, and a contributor to why women's resumes are often weaker: they simply don't get the same opportunities to lead that big project with the cutting edge technology because of unconscious bias.

If you are interested in balancing your team, I recommend the book Women in Tech by Tarah Wheeler Van Vlack. It's meant for women (American specifically), but will be enlightening to you as well, assuming you are male. As a female programmer, I found it both devastating for what's stacked against me and hopeful that--if I play it right--I can mitigate the penalty of my gender stereotype and advance according to my real skills.

team discussions, team building exercises and team outings

If the minority gender are not participating, then what you have is a set of sessions, which unthinkingly, have become gender bonding sessions.

It will take some research and reading to become aware of just how insidious some of these gender bonding traits are within the types of working, playing and phrasing of staff interactions. There are some (but not enough) good resources to help with becoming more gender neutral and pro-competence (rather than using surrogate indicators that have gender bias).

While there will be local national-identity issues the general trend is the same (to avoid discrimination based on certain characteristics). Ultimately a discriminatory decision will need to be made (hire/no-hire) (that is what a decision is), so make sure you know what the actual competence discriminators are.

For side research look at
[1] https://www.youtube.com/watch?v=jIXIuYdnyyk Tutorial: 21 fairness definitions and their politics, Published on Mar 1, 2018, Arvind Narayanan

[2] Arvind Narayanan: Associate professor of computer science at Princeton https://www.youtube.com/user/33BitsOfEntropy