Treating investor funds depends on how they were raised. Are the investors simply loaning you private money? Those are entered as long term loans same as if you went down to the bank on the corner. Use of the funds is up to you and all you have to do is make interest or interest and principal payments depending on the negotiated (and written terms) In this scenario private money is like gold. It arrives in your hands quicker with usually less strings and paperwork albeit generally a higher interest rate.
Or are the investors buying in to your LLC for partial memberships and will receive not only interest on loaned money but a share of the profits when you sell? This is more complicated and better suited for longer term investments buying-improving-holding properties for 3-10 years. Investors will be more skeptical in this type of deal if you are new to this. With memberships you will provide more running commentary which includes not only how the project is going but monthly P&L to investors who are now part-owners
Nearly all the funds you spend on a flip, from acquisition to improvements to and including costs of eventual sale are all non-expenses until you sell. This is probably the number one thing about flips that gets glossed over when promoting flips. All of it is posted to just work in progress (WIP) and you set up a WIP current asset account to post everything to, including labor and materials. I believe (but am open to correction) you can deduct taxes, insurance, maybe some utilities that are required to keep structure sound and secure. You can take depreciation on the originating building asset value on your books but if it is only a year of depreciation it is probably not worth the paperwork since this is only on purchase as improvements cannot be expensed, as I said, until you sell (or place into rental service)
Once you sell you transfer the WIP including acquisition asset value into the sale transaction against sale price to determine your capital gain or loss.
A final caution - if you flip this property from acquisition to sale in less than 12 months then your gain is short term capital gain and probably taxed as regular income unless you qualify as a broker and since this is your first rodeo you would not. Here is just one article backing up what I have written. https://budgeting.thenest.com/tax-rules-purchasing-flip-home-31549.html