What more should I add when applying for a Standard Visitor visa to attend British Army assessment?

You need a UK entry clearance in order to get your army assessment. This is a two day affair that comes midway in the recruitment process. Other than sending an invitation the army does nothing else. Applicants are expected to pay their own expenses and for those needing it, acquire an entry clearance. UKVI treats the occasion as a job interview and so the entry clearance needed is a Standard Visitor Visa issued under Appendix V of the rules.

Nearly every facet of your personal circumstances is alarmingly weak and moreover you have a prior refusal to contend with. Deeply worrying is your intent to provide receipts from Western Union and Moneygram as evidence of your solvency. This is a money laundering pattern, even for small amounts, and they are not going to like this at all. See My application was refused due to 'lack of evidence of funds', are there hidden requirements for UK visa applications?.

If they do not refuse outright on funds provenance, you can show few if any, ties to Ghana and no reason whatsoever to return at the conclusion of your visit. This is usually the point where we advise people to wait for a more prosperous time in their lives before applying for a UK visa.

In the comments you revealed that you have three sisters who are career soldiers in the British army. This is gold. And in your case, solid gold. It gives you a fully defensible premise that they will not be able to rebut. You come from an army family and it's natural for you to join. If played correctly your premise can be put forward in a way that finesses your other weaknesses. This is not a certain bet, but it opens a pathway in what's otherwise a doomed application. You'll need to do more than just mentioning it in your application.

Unfortunately we are not in the business of providing legal advice and hence would not be able to provide specific help with your application. Our domain is travel and visas are a subsidiary topic. You can try on our collateral cousin's site 'law', but I suspect they would be reluctant to provide specific help for a situation as complex as yours.

You can get help from a solicitor, or in this particular case from a seasoned adviser. Both of these attract a fee for their services and you would not be able to get help for free. I would estimate a solicitor would charge about GBP 800 and an adviser about GBP 350 - 400 (at commensurate quality) but these are just onageristic guesses and may not reflect the current market for immigration services.

Avoid using visa agents in Ghana because you may not be able to detect if you are being scammed or not. And avoid internet forums because your situation is too complex and too delicate to be entrusting it to random people on the net.