Thew best process is that in y ear 2 or 3 of your history degree you apply for law vacation schemes and training contracts as firms recruit well ahead. If you are lucky enough to get one who sponsors your post grad they will take care of all that - eg City Consortium firms pay for a PGDL and SQE 1 and 2 course for their future trainees. To get those you need too exams, of course also put all the hard work into applications and do law things at niversity eg join law society, try to get involved in legal things, ideally some time in a law firm just seeing what it is like.
If you cannot get a firm to pay then you have the difficult decision of whether to do an SQE course without a law conversion first - I think it is better to do the law conversion although not strictly necessary for solicitors now (barristers must do it - different route)., You could get a student finance masters loan for a PGDL/SQE masters course
80% of future solicitors use BPP or University of Law for their post grad work including just about all leading law firms so you are probably better of using them for the post grad law.
Law has a kind of UCAS for lawyers for post grad called the Central Applications Board. you can apply from about Sept of year 3 of your degree or later that academic year and most undergraduates get places - it is just a question of who can pay the fees, rather than competing to apply.
The SQE system is quite complicated but above is the basics. In theory you can do your 2 years of on job training/training contract even with voluntary work in law now BUT (massive but) cobbled together not very good 2 years of training is not going to help you get a newly qualified job.